[Bug]: Python glob fallback mutates the process-wide working directory

Bug Description

The Python fallback in openhands-tools/openhands/tools/glob/impl.py:185-231 implements relative matching by calling os.chdir(search_path) and restoring it afterward.

cwd is process-global, so concurrent executor use can race with unrelated threads or any code using relative paths. The tool consequently declares the fallback unsafe for parallel execution and forces tool-wide serialization (glob/definition.py:68-82). Direct executor callers are still exposed to the race.

Expected Behavior

Glob searches should be independent of process-global state and safe to run concurrently regardless of whether ripgrep is installed.

Actual Behavior

An independent, self-contained reproducer is posted in the issue comments. Save
it as /tmp/repro_4663.py, then run:

uv run python /tmp/repro_4663.py

On SDK commit bc5dfc50d, it exercises the real Python glob fallback over 20,000 files; a process signal samples CWD without replacing or delaying glob and prints:

fallback_selected=True
process_cwd_changed=True
process_cwd_restored=True
result_count=100

Suggested Implementation

Use Python's supported glob(..., root_dir=search_path, recursive=True) and join returned relative paths to search_path without resolving symlinks. Both backends can then declare no exclusive resource.

Acceptance Criteria

• Forced-fallback searches across two roots can run concurrently.
• Process cwd never changes during fallback execution.
• Ripgrep/fallback parity remains for **, hidden paths, absolute patterns, symlinks, modification-time ordering, and truncation.
• Tool action/observation schemas remain unchanged.

Version

Current main at 6d3881035. Existing issue search found no duplicate.

Start with the Python fallback in openhands-tools/openhands/tools/glob/impl.py:185-231 and the serialization logic in glob/definition.py:68-82. Run /tmp/repro_4663.py with uv run python to observe the process-wide CWD change. Done means concurrent forced-fallback searches leave CWD unchanged while preserving backend parity and existing schemas.