Fix: pymysql.err.InterfaceError: (0, '') during long time streaming chat responses (#6548) (#7057)

### Related Issue:
https://github.com/infiniflow/ragflow/issues/6548

### Related PR:
https://github.com/infiniflow/ragflow/pull/6861


### Environment:
Commit version:
[[48730e0](48730e00a8)]

### Bug Description:
Unexpected `pymysql.err.InterfaceError: (0, '') `when using Peewee +
PyMySQL + PooledMySQLDatabase after a long-running `chat streamly`
operation.

This is a common issue with Peewee + PyMySQL + connection pooling: you
end up using a connection that was silently closed by the server, but
Peewee doesn't realize it's dead.

**I found that the error only occurs during longer streaming outputs**
and is unrelated to the database connection context, so it's likely
because:

- The prolonged streaming response caused the database connection to
time out

- The original database connection might have been disconnected by the
server during the streaming process

### Why This Happens
This error happens even when using `@DB.connection_context() `after the
stream is done. After investigation, I found this is caused by MySQL
connection pools that appear to be open but are actually dead (expired
due to` wait_timeout`).

1. `@DB.connection_context()` (as a decorator or context manager) pulls
a connection from the pool.

2. If this connection was idle and expired on the MySQL server (e.g.,
due to `wait_timeout`), but not closed in Python, it will still be
considered “open” (`DB.is_closed() == False`).

3. The real error will occur only when I execute a SQL command (such as
.`get_or_none()`), and PyMySQL tries to send it to the server via a
broken socket.


### Changes Made:

1. I implemented manual connection checks before executing SQL:
```
    try:
        DB.execute_sql("SELECT 1")
    except Exception:
        print("Connection dead, reconnecting...")
        DB.close()
        DB.connect()
```
2. Delayed the token count update until after the streaming response is
completed to ensure the streaming output isn't interrupted by database
operations.
```
        total_tokens = 0 
        for txt in chat_streamly(system, history, gen_conf):
            if isinstance(txt, int):
                total_tokens = txt
......
                break
......
        if total_tokens > 0:
            if not TenantLLMService.increase_usage(self.tenant_id, self.llm_type, txt, self.llm_name):
                logging.error("LLMBundle.chat_streamly can't update token usage for {}/CHAT llm_name: {}, content: {}".format(self.tenant_id, self.llm_name, txt))
```

api/db/services/llm_service.py

     @classmethod
     @DB.connection_context()
     def increase_usage(cls, tenant_id, llm_type, used_tokens, llm_name=None):
+        try:
+            if not DB.is_connection_usable():
+                DB.connect()
+        except Exception:
+            DB.close()
+            DB.connect()
         e, tenant = TenantService.get_by_id(tenant_id)
         if not e:
             logging.error(f"Tenant not found: {tenant_id}")
 
         ans = ""
         chat_streamly = self.mdl.chat_streamly
-
+        total_tokens = 0 
         if self.is_tools and self.mdl.is_tools:
             chat_streamly = self.mdl.chat_streamly_with_tools
 
         for txt in chat_streamly(system, history, gen_conf):
             if isinstance(txt, int):
+                total_tokens = txt
                 if self.langfuse:
                     generation.end(output={"output": ans})
-
-                if not TenantLLMService.increase_usage(self.tenant_id, self.llm_type, txt, self.llm_name):
-                    logging.error("LLMBundle.chat_streamly can't update token usage for {}/CHAT llm_name: {}, content: {}".format(self.tenant_id, self.llm_name, txt))
-                return ans
+                break
 
             if txt.endswith("</think>"):
                 ans = ans.rstrip("</think>")
 
             ans += txt
             yield ans
+        if total_tokens > 0:
+            if not TenantLLMService.increase_usage(self.tenant_id, self.llm_type, txt, self.llm_name):
+                logging.error("LLMBundle.chat_streamly can't update token usage for {}/CHAT llm_name: {}, content: {}".format(self.tenant_id, self.llm_name, txt))