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