fix: handle FileNotFoundError in _get_file_fixes when files are missing on disk

[sentry]

During coverage uploads, _produce_file_fixes attempts to open each relevant source file returned by the versioning system (e.g. git ls-files). If a file is tracked in git but missing from the working tree (for example, deleted locally or omitted via sparse checkout), open() raises FileNotFoundError, which previously crashed the entire upload.

This change adds an except FileNotFoundError: handler to _get_file_fixes alongside UnicodeDecodeError and IsADirectoryError. When encountered, a warning is logged and an empty fixer entry is returned, allowing the upload to complete successfully.

Fixes CLI-HW

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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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