fix: resolve file paths correctly for file fixes

[sentry]

This PR addresses a FileNotFoundError occurring during the file fixing process in codecov_cli/services/upload/upload_collector.py.

Root Cause:
Previously, the _get_file_fixes() method would attempt to open files using paths returned by git ls-files. These paths are relative to the repository root (network_root_folder). However, the CLI process's current working directory (CWD) is not always guaranteed to be the network_root_folder. When the CWD differed, open(filename, 'r') would fail to locate the file, leading to a FileNotFoundError.

Solution:

1. Path Resolution: Modified _get_file_fixes() to construct an absolute path by prepending self.network_finder.network_root_folder to the filename before attempting to open it. This ensures the file is always looked for in the correct location relative to the repository root.
2. Error Handling: Added specific FileNotFoundError handling to gracefully log a warning and skip applying file fixes for that particular file if it cannot be found. This prevents the entire upload process from crashing due to a missing file, similar to how UnicodeDecodeError and IsADirectoryError are handled.

This fix ensures that file fixes are correctly applied in various CI environments where the CWD might not align with the repository root.

Fixes CLI-94

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[sentry]

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