check_bearer_token now uses hmac.compare_digest for constant-time comparison
and rejects a falsy expected token outright, with a startup guard that fails
loudly if HAVOC_MCP_TOKEN is set but empty. Adds Starlette TestClient coverage
of the _BearerAuthMiddleware auth-enforcement path (no header, wrong token,
correct token), and cleans up dead code found during review: unused token
param on build_mcp_server, a per-request import, and an unused json import.
