feat(mcp): generic MCP tool source with per-node function filtering (#301)

* feat(mcp): generic MCP tool source with per-node function filtering

Adds a Model Context Protocol tool category: connect a customer MCP
server and expose its tools to the agent, with optional per-node
allow-listing of individual MCP functions.

- ToolCategory.MCP enum + alembic migration
- MCP definition validator and collision-safe function-name namespacing
- McpToolSession wrapper: graceful-degrade, per-call open/close lifecycle
- CustomToolManager MCP branch (schemas + proxy handlers)
- Per-node mcp_tool_filters threaded through DTO/graph/engine
- Best-effort discovered_tools catalog cache + POST /tools/{uuid}/mcp/refresh
- UI: MCP create/edit config, tabbed ToolSelector with per-node toggles

* feat: refactor for code standardisation and documentation

---------

Co-authored-by: Abhishek Kumar <abhishek@a6k.me>

api/mcp_server/auth.py

@@ -18,7 +18,9 @@ async def authenticate_mcp_request() -> UserModel:
     the `langfuse.user.id` / `langfuse.session.id` attributes make the
     span filterable in the Langfuse UI.
     """
-    headers = get_http_headers()
+    # FastMCP strips Authorization by default unless explicitly included.
+    # Preserve it here so Bearer API keys work for MCP tool invocations.
+    headers = get_http_headers(include={"authorization"})
     api_key = headers.get("x-api-key")
     if not api_key:
         auth = headers.get("authorization", "")