### What problem does this PR solve?

Expand the capabilities of the MCP Server. #8644.

Special thanks to @Drasek, this change is largely based on his original
implementation, it is super neat and well-structured to me. I basically
just integrated his code into the codebase with minimal modifications.

My main contribution is implementing a proper cache layer for dataset
and document metadata, using the LRU strategy with a 300s ± random 30s
TTL. The original code did not actually perform caching.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Caspar Armster <caspar@armster.de>

### What problem does this PR solve?

Using the mcp server in n8n sometimes (with smaller models) results in
errors because the llm misses a char or adds one to the list of
dataset_ids provided. It first asks for the list of datasets and if you
got a larger list of them it makes a error recalling the list
completely. So adding the feature to just search through all available
datasets solves this and makes the retrieval of data more stable. The
functionality to just call special datasets by id is not changed, the
dataset_ids are now not required anymore (only the "question" is). You
can provide (like before) a list of datasets, a empty list or no list at
all.

### Type of change

- [X] New Feature (non-breaking change which adds functionality)
<img width="1897" height="880" alt="mcp error dataset id"
src="https://github.com/user-attachments/assets/71076d24-f875-4663-a69a-60839fc7a545"
/>

### What problem does this PR solve?

Enable MCP streamable-http model via docker compose

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Add MCP treamable-http transport.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Upgrade MCP SDK to v1.9.4 (latest).

### Type of change

- [x] Refactoring

### What problem does this PR solve?

Add authorization header for MCP server based on [OAuth
2.1](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-12#section-5).

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Add mcp self-host mode, a complement of #7084.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Add MCP support with a client example.

Issue link: #4344

### Type of change

- [x] New Feature (non-breaking change which adds functionality)