fix: discard generated segment output that has no probeable duration

[seonghobae]

Problem

_execute_segment_conversion validates a freshly generated segment output by probing its duration:

output_duration = _probe_output_duration(first_result, ...)

probe_media (via _parse_probe_payload) raises MediaShrinkerError when the output has no usable/zero duration or no audio stream. ffmpeg can exit 0 yet still produce a truncated/empty output (e.g. a degenerate last segment, or a -ss past the end). In that case the exception propagated straight out of the conversion:

• The invalid generated file was left orphaned on disk — unlike the too_large / too_long / duration_mismatch cases, which all discard the file.
• Worse, on the next run _find_valid_existing_output re-probes that leftover file, raises again, and the corrupt output poisons the source permanently (it is never cleaned up).

This matches two of the audited failure modes: a duration parse that mishandles missing/zero, and discard logic that leaves an invalid file.

Fix

Guard the probe with a MediaShrinkerError handler that discards the invalid output via the existing _discard_invalid_generated_output helper and returns a graceful duration_mismatch result (output_path=None, duration_seconds=None). Minimal, localized to the duration-validation region.

Test

Added test_convert_segment_discards_output_that_has_no_probeable_duration, which fakes an ffmpeg that writes a file the probe cannot read a duration for. It fails on the pre-fix code (raises MediaShrinkerError, file left on disk) and passes after the fix (status duration_mismatch, file removed).

Gates

• coverage on media_shrinker.py: 100%
• interrogate: 100%
• Full suite: 114 tests, only the 5 pre-existing macOS os.listxattr errors (baseline) — no new failures.

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