feat: implement mcp frontend

src/beaver_gateway/mcp/client_pool.py

@@ -16,8 +16,11 @@ from fastmcp import Client
 from fastmcp.client.transports import StdioTransport, StreamableHttpTransport
 from fastmcp.server import create_proxy
 
+from beaver_gateway.mcp.lenient import LenientStdioTransport
+
 if TYPE_CHECKING:
     from fastmcp import FastMCP
+    from fastmcp.client.transports.base import ClientTransport
 
     from beaver_gateway.mcp.types import HttpMcp, StdioMcp
 
@@ -27,22 +30,37 @@ def build_stdio_proxy(spec: StdioMcp) -> FastMCP:
 
     ``spec.command`` is a non-empty tuple; the first element is the
     executable and the rest are CLI args. ``StdioTransport`` keeps the
-    subprocess alive across calls.
+    subprocess alive across calls. When ``spec.lenient`` is set we
+    swap in :class:`LenientStdioTransport`, which silently drops
+    non-JSON-RPC stdout lines instead of forwarding them as exceptions
+    (lets us ingest MCPs that print ``Processing...`` chatter on stdout
+    without leaking warnings to downstream UIs).
     """
     if not spec.command:
         msg = f"stdio MCP {spec.name!r} has empty command"
         raise ValueError(msg)
     command, *args = spec.command
-    transport = StdioTransport(
-        command=command,
-        args=list(args),
-        env=spec.env,
-        cwd=str(spec.cwd) if spec.cwd is not None else None,
-    )
+    cwd = str(spec.cwd) if spec.cwd is not None else None
+    transport: ClientTransport
+    if spec.lenient:
+        transport = LenientStdioTransport(
+            command=command, args=list(args), env=spec.env, cwd=cwd
+        )
+    else:
+        transport = StdioTransport(
+            command=command, args=list(args), env=spec.env, cwd=cwd
+        )
     return create_proxy(Client(transport, name=spec.name))
 
 
 def build_http_proxy(spec: HttpMcp) -> FastMCP:
-    """Wrap a remote streamable-HTTP MCP into a mountable ``FastMCPProxy``."""
-    transport = StreamableHttpTransport(url=spec.url, auth=spec.auth)
+    """Wrap a remote streamable-HTTP MCP into a mountable ``FastMCPProxy``.
+
+    ``spec.headers`` are folded into the upstream client; pass per-call
+    auth (e.g. ``X-Api-Key``) here rather than as ``auth`` when the
+    upstream rejects Bearer headers.
+    """
+    transport = StreamableHttpTransport(
+        url=spec.url, auth=spec.auth, headers=spec.headers
+    )
     return create_proxy(Client(transport, name=spec.name))