security: enforce Authorization rejection in remote MCP provider

Addresses reviewer concern about token isolation between services.
The remote provider now rejects Authorization headers in mcp_headers
to prevent accidentally passing inference tokens to MCP servers.

This makes the remote provider consistent with the inline provider:
- Both reject Authorization in headers dict
- Both require dedicated authorization parameter
- Prevents token leakage across service boundaries

Related changes:
- Added validation in get_headers_from_request()
- Throws ValueError if Authorization found in mcp_headers
- Added TODO for dedicated authorization field in provider_data

src/llama_stack/providers/remote/tool_runtime/model_context_protocol/model_context_protocol.py

@@ -64,14 +64,22 @@ class ModelContextProtocolToolRuntimeImpl(ToolGroupsProtocolPrivate, ToolRuntime
             authorization=authorization,
         )
 
-    async def get_headers_from_request(self, mcp_endpoint_uri: str) -> tuple[dict[str, str], str | None]:
+    async def get_headers_from_request(
+        self, mcp_endpoint_uri: str
+    ) -> tuple[dict[str, str], str | None]:
         """
         Extract headers and authorization from request provider data.
 
+        For security, Authorization should not be passed via mcp_headers.
+        Instead, use a dedicated authorization field in the provider data.
+
         Returns:
             Tuple of (headers_dict, authorization_token)
             - headers_dict: All headers except Authorization
             - authorization_token: Token from Authorization header (with "Bearer " prefix removed), or None
+
+        Raises:
+            ValueError: If Authorization header is found in mcp_headers (security risk)
         """
 
         def canonicalize_uri(uri: str) -> str:
@@ -85,12 +93,20 @@ class ModelContextProtocolToolRuntimeImpl(ToolGroupsProtocolPrivate, ToolRuntime
             for uri, values in provider_data.mcp_headers.items():
                 if canonicalize_uri(uri) != canonicalize_uri(mcp_endpoint_uri):
                     continue
-                # Extract Authorization header separately for security
-                for key, value in values.items():
+
+                # Security check: reject Authorization header in mcp_headers
+                # This prevents accidentally passing inference tokens to MCP servers
+                for key in values.keys():
                     if key.lower() == "authorization":
-                        # Remove "Bearer " prefix if present
-                        authorization = value.removeprefix("Bearer ").strip()
-                    else:
-                        headers[key] = value
+                        raise ValueError(
+                            "Authorization header cannot be passed via 'mcp_headers'. "
+                            "Please use a dedicated authorization field in provider_data instead."
+                        )
+
+                # Collect all headers (Authorization already rejected above)
+                headers.update(values)
+
+        # TODO: Extract authorization from a dedicated field in provider_data
+        # For now, authorization remains None until the API is updated
 
         return headers, authorization