fix: handle FileNotFoundError for temporary coverage files

[sentry]

This PR addresses a FileNotFoundError that occurs when the Codecov CLI attempts to read temporary coverage files (e.g., .tmp/coverage-*.json from Jest/Istanbul) that have been deleted by the test framework before the CLI can process them.

The root cause is a TOCTOU (Time-of-Check-Time-of-Use) race condition: the CLI discovers these files, but they are removed from the filesystem before their content can be read.

To fix this, the _format_file method in upload_sender.py now includes a try-except FileNotFoundError block. If a file is not found during content reading, a warning is logged, and the file is gracefully skipped from the upload payload. The _get_files method has been updated to filter out these skipped (None) entries.

Fixes CLI-48

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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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