Make log trimming failures nonfatal and bounded

[AvdLee]

Summary

• Fixes the debug-build crash Fatal error: Could not write trimmed log to target file location: .../diagnostics_log.txt by removing the assertionFailure paths in LogsWriter — trimming failures are now nonfatal.
• Surfaces the previously swallowed atomic-write error via os_log with the full NSError (with a dedicated message for NSFileWriteOutOfSpaceError, the most plausible field cause since the atomic replace needs temp space for the full trimmed file). print is intentionally avoided because stdout/stderr are piped back into the diagnostics logger.
• Closes the append FileHandle before trimming reads and atomically replaces the same file.
• Trims oversized logs fully below the configured limit in one pass instead of removing only ten records per write, which previously left the file over the limit and retried the atomic replacement on every subsequent write.

Test plan

• New regression tests: large overflow trimmed below the limit, repeated threshold crossings stay bounded, a single record larger than the limit doesn't trap, and a forced atomic-write failure (read-only parent directory) neither terminates nor loses the appended record.
• New logger-level test exercising trimming through the serial queue at the production 2 MB limit with concurrent readLog() calls.
• Full package test suite passes (56 tests, 0 failures).