Fix: make clang-tidy lint resilient to missing or broken compile databases

[chenshengxin2026]

Summary

• _load_compile_database now returns None instead of crashing when a compile_commands.json is missing, contains invalid JSON, or cannot be reconfigured
• _build_file_index skips None entries gracefully so a single broken target does not block the entire lint run
• Adds from __future__ import annotations for tuple[...] | None syntax compatibility

Split out from #642 (UT reorganization) — that PR no longer touches tests/lint/clang_tidy.py.

Test plan

• Verified clang_tidy.py still works with valid compile databases (no behavioral change on happy path)
• Verified the if not file_index guard (line 194-196) still catches the all-databases-broken case and returns exit code 1

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request enhances the error handling in tests/lint/clang_tidy.py when loading compilation databases. Key changes include adding from __future__ import annotations, updating _load_compile_database to return None upon failure (e.g., FileNotFoundError or reconfiguration errors), and modifying _build_file_index to skip entries where the database could not be loaded. I have no feedback to provide.