fix: Highly inconsistent agent response to cogito agent calling MCP server - Body "Invalid http method" (#7790)

* fix: resolve duplicate MCP route registration causing 50% failure rate

Fixes #7772

The issue was caused by duplicate registration of the MCP endpoint
/mcp/v1/chat/completions in both openai.go and localai.go, leading
to a race condition where requests would randomly hit different
handlers with incompatible behaviors.

Changes:
- Removed duplicate MCP route registration from openai.go
- Kept the localai.MCPStreamEndpoint as the canonical handler
- Added all three MCP route patterns for backward compatibility:
  * /v1/mcp/chat/completions
  * /mcp/v1/chat/completions
  * /mcp/chat/completions
- Added comments to clarify route ownership and prevent future conflicts
- Fixed formatting in ui_api.go

The localai.MCPStreamEndpoint handler is more feature-complete as it
supports both streaming and non-streaming modes, while the removed
openai.MCPCompletionEndpoint only supported synchronous requests.

This eliminates the ~50% failure rate where the cogito library would
receive "Invalid http method" errors when internal HTTP requests were
routed to the wrong handler.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: majiayu000 <1835304752@qq.com>

* Address feedback from review

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

---------

Signed-off-by: majiayu000 <1835304752@qq.com>
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>