Docs: add curl example for interacting with the RAGFlow MCP server (#8372)

### What problem does this PR solve?

Add curl example for interacting with the RAGFlow MCP server. Special
thanks to @writinwaters for his expert refinement.

### Type of change

- [x] Documentation Update

---------

Co-authored-by: writinwaters <cai.keith@gmail.com>

docs/develop/mcp/mcp_client_example.md

@@ -1,22 +1,240 @@
 ---
 sidebar_position: 3
 slug: /mcp_client
+
 ---
 
-# RAGFlow MCP client example
+# RAGFlow MCP client examples
+
+Python and curl MCP client examples.
+
+------
+
+## Example MCP Python client
 
 We provide a *prototype* MCP client example for testing [here](https://github.com/infiniflow/ragflow/blob/main/mcp/client/client.py).
 
 :::danger IMPORTANT
-If your MCP server is running in host mode, include your acquired API key in your client's `headers` as shown below:
+If your MCP server is running in host mode, include your acquired API key in your client's `headers` when connecting asynchronously to it:
+
 ```python
 async with sse_client("http://localhost:9382/sse", headers={"api_key": "YOUR_KEY_HERE"}) as streams:
     # Rest of your code...
 ```
 
-Or follow the requirements of [OAuth 2.1 Section 5](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-12#section-5) by providing an Authorization request headers field:
+Alternatively, to comply with [OAuth 2.1 Section 5](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-12#section-5), you can run the following code *instead* to connect to your MCP server:
+
 ```python
 async with sse_client("http://localhost:9382/sse", headers={"Authorization": "YOUR_KEY_HERE"}) as streams:
     # Rest of your code...
 ```
 :::
+
+## Use curl to interact with the RAGFlow MCP server
+
+When interacting with the MCP server via HTTP requests, follow this initialization sequence:
+
+1. **The client sends an `initialize` request** with protocol version and capabilities.
+2. **The server replies with an `initialize` response**, including the supported protocol and capabilities.
+3. **The client confirms readiness with an `initialized` notification**.
+   _The connection is established between the client and the server, and further operations (such as tool listing) may proceed._
+
+:::tip NOTE
+For more information about this initialization process, see [here](https://modelcontextprotocol.io/docs/concepts/architecture#1-initialization). 
+:::
+
+In the following sections, we will walk you through a complete tool calling process.
+
+### 1. Obtain a session ID
+
+Each curl request with the MCP server must include a session ID:
+
+```bash
+$ curl -N -H "api_key: YOUR_API_KEY" http://127.0.0.1:9382/sse
+```
+
+:::tip NOTE
+See [here](../acquire_ragflow_api_key.md) for information about acquiring an API key.
+:::
+
+#### Transport
+
+The transport will stream messages such as tool results, server responses, and keep-alive pings.
+
+_The server returns the session ID:_
+
+```bash
+event: endpoint
+data: /messages/?session_id=5c6600ef61b845a788ddf30dceb25c54
+```
+
+### 2. Send an `Initialize` request
+
+The client sends an `initialize` request with protocol version and capabilities:
+
+```bash
+session_id="5c6600ef61b845a788ddf30dceb25c54" && \
+
+curl -X POST "http://127.0.0.1:9382/messages/?session_id=$session_id" \
+  -H "api_key: YOUR_API_KEY" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "initialize",
+    "params": {
+      "protocolVersion": "1.0",
+      "capabilities": {},
+      "clientInfo": {
+        "name": "ragflow-mcp-client",
+        "version": "0.1"
+      }
+    }
+  }' && \
+```
+
+#### Transport
+
+_The server replies with an `initialize` response, including the supported protocol and capabilities:_
+
+```bash
+event: message
+data: {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2025-03-26","capabilities":{"experimental":{"headers":{"host":"127.0.0.1:9382","user-agent":"curl/8.7.1","accept":"*/*","api_key":"ragflow-xxxxxxxxxxxx","accept-encoding":"gzip"}},"tools":{"listChanged":false}},"serverInfo":{"name":"ragflow-server","version":"1.9.4"}}}
+```
+
+### 3. Acknowledge readiness
+
+The client confirms readiness with an `initialized` notification:
+
+```bash
+curl -X POST "http://127.0.0.1:9382/messages/?session_id=$session_id" \
+  -H "api_key: YOUR_API_KEY" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "method": "notifications/initialized",
+    "params": {}
+  }' && \
+```
+
+ _The connection is established between the client and the server, and further operations (such as tool listing) may proceed._
+
+### 4. Tool listing
+
+```bash
+curl -X POST "http://127.0.0.1:9382/messages/?session_id=$session_id" \
+  -H "api_key: YOUR_API_KEY" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "id": 3,
+    "method": "tools/list",
+    "params": {}
+  }' && \
+```
+
+#### Transport
+
+```bash
+event: message
+data: {"jsonrpc":"2.0","id":3,"result":{"tools":[{"name":"ragflow_retrieval","description":"Retrieve relevant chunks from the RAGFlow retrieve interface based on the question, using the specified dataset_ids and optionally document_ids. Below is the list of all available datasets, including their descriptions and IDs. If you're unsure which datasets are relevant to the question, simply pass all dataset IDs to the function.","inputSchema":{"type":"object","properties":{"dataset_ids":{"type":"array","items":{"type":"string"}},"document_ids":{"type":"array","items":{"type":"string"}},"question":{"type":"string"}},"required":["dataset_ids","question"]}}]}}
+
+```
+
+### 5. Tool calling
+
+```bash
+curl -X POST "http://127.0.0.1:9382/messages/?session_id=$session_id" \
+  -H "api_key: YOUR_API_KEY" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "id": 4,
+    "method": "tools/call",
+    "params": {
+      "name": "ragflow_retrieval",
+      "arguments": {
+        "question": "How to install neovim?",
+        "dataset_ids": ["DATASET_ID_HERE"],
+        "document_ids": []
+      }
+
+```
+
+#### Transport
+
+```bash
+event: message
+data: {"jsonrpc":"2.0","id":4,"result":{...}}
+
+```
+
+### A complete curl example
+
+```bash
+session_id="YOUR_SESSION_ID" && \
+
+# Step 1: Initialize request
+curl -X POST "http://127.0.0.1:9382/messages/?session_id=$session_id" \
+  -H "api_key: YOUR_API_KEY" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "initialize",
+    "params": {
+      "protocolVersion": "1.0",
+      "capabilities": {},
+      "clientInfo": {
+        "name": "ragflow-mcp-client",
+        "version": "0.1"
+      }
+    }
+  }' && \
+
+sleep 2 && \
+
+# Step 2: Initialized notification
+curl -X POST "http://127.0.0.1:9382/messages/?session_id=$session_id" \
+  -H "api_key: YOUR_API_KEY" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "method": "notifications/initialized",
+    "params": {}
+  }' && \
+
+sleep 2 && \
+
+# Step 3: Tool listing
+curl -X POST "http://127.0.0.1:9382/messages/?session_id=$session_id" \
+  -H "api_key: YOUR_API_KEY" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "id": 3,
+    "method": "tools/list",
+    "params": {}
+  }' && \
+
+sleep 2 && \
+
+# Step 4: Tool call
+curl -X POST "http://127.0.0.1:9382/messages/?session_id=$session_id" \
+  -H "api_key: YOUR_API_KEY" \
+  -H "Content-Type: application/json" \
+  -d '{
+    "jsonrpc": "2.0",
+    "id": 4,
+    "method": "tools/call",
+    "params": {
+      "name": "ragflow_retrieval",
+      "arguments": {
+        "question": "How to install neovim?",
+        "dataset_ids": ["DATASET_ID_HERE"],
+        "document_ids": []
+      }
+    }
+  }'
+
+```