fix: gracefully handle missing files during upload

[sentry]

This PR addresses issue CLI-3E, where the Codecov CLI would crash with a FileNotFoundError if a coverage file, discovered during the search phase, was no longer present or accessible when its content was being read for upload.

The root cause was a TOCTOU (Time-of-Check to Time-of-Use) race condition: a file like coverage.json might be found by the file search, but then deleted or become a broken symlink before the upload process could read its content. The previous implementation in _get_files in upload_sender.py did not handle this scenario, leading to an unhandled exception and a failed upload.

Changes Made:

• Modified the _get_files method in codecov_cli/services/upload/upload_sender.py.
• Replaced the list comprehension with an explicit loop to process each file individually.
• Wrapped the file content reading (self._format_file(file)) in a try-except FileNotFoundError block.
• If a FileNotFoundError occurs for a specific file, a warning is now logged (e.g., "File not found, skipping: /path/to/missing/file"), and that file is skipped, allowing the upload of other valid files to proceed without interruption.

Fixes CLI-3E

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[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
723	1	722	1

View the full list of 1 ❄️ flaky test(s)

api.temp.calculator.test_calculator::test_divide

Flake rate in main: 100.00% (Passed 0 times, Failed 1726 times)

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

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