feat: add MCP server module exposing search_regulations tool

- New backend/app/mcp/ module: MCPServer instance with a single
  search_regulations tool backed by the existing AgentConversationService.
- MCPAuthMiddleware reuses existing JWT auth (no new auth mechanism).
- Mounted at /mcp/ in api/main.py via Streamable HTTP transport; wired the
  MCP session manager into the existing lifespan() via AsyncExitStack
  (app.mount() does not propagate nested ASGI lifespans automatically).
- Fixed a doubled /mcp/mcp path by setting streamable_http_path to "/"
  (MCPServer.streamable_http_app() defaults to registering its own /mcp route).
- Verified end-to-end with the real mcp Python client: list_tools() returns
  search_regulations, auth correctly 401s without or with an invalid token.
- 7 new tests, 76 total (up from 69), all passing.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/mcp/__init__.py

@@ -0,0 +1,8 @@
+"""MCP (Model Context Protocol) server module.
+
+Exposes selected read-only platform capabilities — currently only regulation
+search — as MCP tools so external MCP clients (Claude Desktop, GitHub Copilot,
+Cursor, etc.) can query this platform's compliance knowledge base directly.
+"""
+# Kept deliberately empty beyond this docstring — see server.py for the
+# actual FastMCP instance and tool/middleware definitions.

backend/app/mcp/server.py

@@ -0,0 +1,88 @@
+"""MCPServer instance exposing the compliance knowledge base as an MCP tool.
+
+This module is a pure protocol adapter: search_regulations() below calls the
+existing AgentConversationService.ask() (the same application service backing
+the /api/v1/agent/ask REST endpoint) and reshapes its result into a plain
+dict. No new retrieval, ranking, or LLM orchestration logic lives here.
+"""
+
+from __future__ import annotations
+
+from mcp.server import MCPServer
+from starlette.responses import PlainTextResponse
+from starlette.types import ASGIApp, Receive, Scope, Send
+
+from app.config.settings import settings
+from app.shared.bootstrap import get_agent_conversation_service, get_jwt_handler
+
+# Single shared MCPServer instance — analogous to the single shared FastAPI
+# `app` instance in app/api/main.py. Tools registered via @mcp.tool() below.
+# Note: the installed mcp SDK (2.0.0) renamed the older "FastMCP" class to
+# "MCPServer" (mcp.server.mcpserver.MCPServer); the .tool()/.streamable_http_app()
+# API surface used here is unchanged across that rename.
+mcp = MCPServer("ai-regulations")
+
+
+@mcp.tool()
+def search_regulations(query: str, top_k: int = 5) -> dict:
+    """Search the compliance knowledge base and return a grounded answer.
+
+    query: Natural-language search question, e.g. "国六排放标准最新要求".
+    top_k: Maximum number of cited sources to return (default 5).
+    """
+    # No session_id is passed: this keeps each call stateless (no
+    # ConversationStore reads/writes), matching "search" semantics rather
+    # than multi-turn chat semantics.
+    _, result = get_agent_conversation_service().ask(query=query, top_k=top_k)
+    return {
+        "answer": result.answer,
+        "sources": [source.__dict__ for source in result.sources],
+    }
+
+
+class MCPAuthMiddleware:
+    """Reject unauthenticated requests before they reach the MCP protocol handler.
+
+    Mirrors the existing get_current_user dependency's behavior (auth.py) but
+    implemented as raw ASGI middleware, since the mounted MCP app is a plain
+    ASGI app, not a FastAPI/APIRouter instance that supports Depends().
+    """
+
+    def __init__(self, app: ASGIApp) -> None:
+        """Store the wrapped ASGI app to delegate to once auth passes."""
+        self.app = app
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        """Validate the bearer token for HTTP requests; pass non-HTTP scopes through."""
+        # Only HTTP requests carry an Authorization header to check; lifespan
+        # and other scope types must always pass through untouched.
+        if scope["type"] != "http" or not settings.auth_enabled:
+            await self.app(scope, receive, send)
+            return
+
+        headers = dict(scope["headers"])
+        auth_header = headers.get(b"authorization", b"").decode()
+        token = auth_header.removeprefix("Bearer ").strip()
+        try:
+            get_jwt_handler().decode_token(token)
+        except ValueError as exc:
+            # Reject before the MCP session/protocol layer ever sees the request.
+            response = PlainTextResponse(str(exc), status_code=401)
+            await response(scope, receive, send)
+            return
+
+        await self.app(scope, receive, send)
+
+
+def build_mcp_asgi_app() -> ASGIApp:
+    """Return the Streamable HTTP ASGI app for the MCP server, auth-guarded.
+
+    streamable_http_path="/" is required here: MCPServer.streamable_http_app()
+    registers its own internal route at "/mcp" by default, and this app is
+    itself mounted at "/mcp" in api/main.py — without overriding the internal
+    path to "/", the effective external path would be the confusing "/mcp/mcp"
+    instead of "/mcp".
+    """
+    asgi_app = mcp.streamable_http_app(streamable_http_path="/")
+    asgi_app.add_middleware(MCPAuthMiddleware)
+    return asgi_app