fix: handle broken symlinks in filewatcher initial scan

[abhu85]

Summary

This PR fixes the file watcher crash when encountering broken symlinks during the initial cache seeding phase.

Problem: When a directory contains a symlink pointing to a non-existent file (such as Emacs lock files like .#app.py), the _seed_mtime_cache method would crash with a FileNotFoundError, making the file watcher unusable and preventing hot-reloading during development.

Solution: Add exception handling to _seed_mtime_cache to catch (OSError, IOError) when calling mtime(), consistent with the existing error handling in _is_changed_file. Files that cannot be stat'd are skipped with a debug log message.

Changes

• chalice/cli/filewatch/stat.py: Added try/except block in _seed_mtime_cache to handle broken symlinks
• tests/unit/cli/filewatch/test_stat.py: Added regression test test_can_handle_broken_symlink_during_initial_scan

Reproduction

# Create a broken symlink (like Emacs creates)
ln -s /nonexistent/path .#app.py
# Run chalice local - this would crash before this fix
chalice local

Test Plan

• Unit tests pass (pytest tests/unit/cli/filewatch/test_stat.py)
• New regression test added that specifically tests broken symlink handling during initial scan
• Manual reproduction confirmed - broken symlinks are now skipped gracefully
• Flake8 passes
• Mypy passes with project configuration
• Pylint score: 10.00/10

Fixes #997

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