#305 Preserve test run history

This PR adds test run history to the final interview Markdown report, allowing candidates to review previous code snapshots, test results, and error messages.

• Record the 10 most recent test runs.
• Save code snapshots only when the code changes to reduce report size.
• Include timestamps, test results, and available error details.
• Preserve existing input handling and the final code report.

Closes #301

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Summary by cubic

Preserves test run history and renders it in the final interview Markdown report so candidates can review earlier code snapshots, test results, and error messages.

• Records the 10 most recent runs with a code snapshot taken at test start, including run number, timestamp, language, pass/fail totals, setup errors, and per-case input, expected, actual, and error details.
• Renders a ## Test Run History section with each run's code snapshot and test case breakdown; the section is omitted when no tests were run.
• Existing input handling and the final code report are unchanged.