fix: resolve file paths correctly in _get_file_fixes

[sentry]

This PR addresses a FileNotFoundError occurring in the _get_file_fixes function within upload_collector.py.

Problem:
The _get_file_fixes function attempted to open files using paths (filename) that were relative to the network_root_folder (as provided by git ls-files), but the open() call was implicitly resolving these paths relative to the current working directory. This led to FileNotFoundError when the current working directory differed from the network_root_folder, a common scenario in various CI environments. Additionally, there was no specific error handling for FileNotFoundError.

Solution:

1. Corrected File Path Resolution: Modified the open() call to explicitly use the absolute path by prepending self.network_finder.network_root_folder to the filename.
2. Updated Pathlib Object: Ensured the pathlib.Path object also uses the correctly resolved absolute path.
3. Added FileNotFoundError Handling: Implemented an except FileNotFoundError block to gracefully handle cases where a file might still not be found (e.g., due to race conditions or external changes), logging a warning instead of crashing the upload process.

Fixes CLI-94