Fix: Handle OSError when git config lock fails on network drives

[SANJAY23CB50]

Problem

When using aider on network drives or with certain file system configurations, the repo.config_writer() call can fail with an OSError due to file locking issues. This causes aider to crash during the git setup phase.

Solution

• Wrapped the config_writer() call in a try-except block to catch OSError and PermissionError
• Added informative warning message that explains the issue and provides manual workaround
• Allows aider to continue in read-only mode instead of crashing
• Added test case to verify the error handling

Testing

• Added tests/basic/test_git_config_network.py with test coverage for the error scenario
• Test mocks the OSError and verifies graceful handling

Fixes #4508