feat(files-mcp): mount at /files-mcp via MCP_SERVICES registry

- main.py: import files_app, append McpService entry to MCP_SERVICES.
  Lifespan already iterates the list, so no edit to the lifespan function.
- tests/conftest.py: autouse fixture rebinds files_mcp.core.path_guard._ROOT
  to a per-test tmp_path so the sandbox is fresh for every test.
- files_mcp/__init__.py: drop the try/except around the server import now
  that server.py exists, restoring fail-fast on a missing/corrupt server
  module in production. The _init_root() try/except stays, since pytest
  collection can import the package before FILES_MCP_ROOT is set.

Verified via uv run pytest: 327 tests pass, no regressions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

files_mcp/__init__.py

@@ -16,13 +16,12 @@ try:
 except RuntimeError:
     _ROOT = None
 
-# Re-export the FastAPI app once the sandbox root has been initialized.
-# server.py is created in a later task; importing it lazily keeps this
-# package importable (and unit-testable) before that task lands.
-try:  # pragma: no cover - exercised once server.py exists
-    from files_mcp.server import app  # noqa: E402
-except ImportError:
-    app = None  # type: ignore[assignment]
+# Re-export the FastAPI app. server.py now exists, so this import is
+# unconditional — a missing/corrupt server.py fails loud at import time
+# (production-correct behavior). The sandbox root is only needed by the
+# tool routes at request time, not at import, so this stays safe to import
+# even when FILES_MCP_ROOT is unset (e.g. during pytest collection).
+from files_mcp.server import app  # noqa: E402
 
 ROOT: Path | None = _ROOT
 

main.py

@@ -10,6 +10,7 @@ from fastapi import FastAPI
 
 from common.factory import init_mcp_server
 from common.logging import logger
+from files_mcp import app as files_app
 from spark_executor import app as spark_executor_app
 
 
@@ -39,6 +40,11 @@ MCP_SERVICES: list[McpService] = [
         app=spark_executor_app,
         mount_path="/spark-executor-mcp",
     ),
+    McpService(
+        name="files",
+        app=files_app,
+        mount_path="/files-mcp",
+    ),
 ]
 
 

tests/conftest.py

@@ -6,3 +6,20 @@ from pathlib import Path
 ROOT = Path(__file__).resolve().parents[1]
 if str(ROOT) not in sys.path:
     sys.path.insert(0, str(ROOT))
+
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _isolate_files_mcp_root(tmp_path, monkeypatch):
+    """Point files_mcp at a fresh tmp_path for every test.
+
+    Sets FILES_MCP_ROOT and rebinds files_mcp.core.path_guard's module-level
+    _ROOT, so each test starts with an empty sandbox. Spark_executor tests
+    are unaffected by this fixture — they don't touch the files_mcp sandbox.
+    """
+    monkeypatch.setenv("FILES_MCP_ROOT", str(tmp_path))
+    from files_mcp.core import path_guard
+    path_guard.set_root_for_testing(tmp_path.resolve())
+    yield