fix(export): tighten native audio safety guards

[meiiie]

Summary

• Reject CUDA inline-audio summaries when an audio stream exists but has no duration metadata.
• Avoid stream-copying unknown source audio codecs into MP4; transcode to AAC instead.
• Add regression coverage for both native audio safety guards.

Why

This is a final small audio-safety slice from #504. Native export should only trust CUDA inline audio when the output audio stream can be validated, and MP4 muxing should not copy unknown sidecar/WAV/PCM-like codecs blindly.

This PR intentionally avoids route planner, CUDA compositor binary, and dev launcher changes.

Verification

• npm test -- electron/ipc/export/native-video.test.ts
• npx tsc --noEmit
• npx biome check --formatter-enabled=false electron/ipc/export/native-video.ts electron/ipc/export/native-video.test.ts
• git diff --check

Summary by CodeRabbit

Bug Fixes

• Fixed audio codec handling during MP4 exports—unknown audio codecs are now properly transcoded to AAC instead of causing potential issues.
• Improved validation for NVIDIA CUDA exports to properly detect missing audio duration data, preventing invalid export configurations.
• Enhanced edge-case handling for missing or unspecified audio codec information during video export operations.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Run ID: ed6ef344-883b-478c-9ad8-b672e283b0a6

📥 Commits

Reviewing files that changed from the base of the PR and between 81e4739 and 591fde8.

📒 Files selected for processing (2)

• electron/ipc/export/native-video.test.ts
• electron/ipc/export/native-video.ts

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📝 Walkthrough

Walkthrough

This PR adds validation refinements for audio codec handling in MP4 muxing and NVIDIA CUDA exports. Empty audio codecs now correctly return false from copy-into-MP4 checks, forcing transcoding. CUDA validation now distinguishes missing audio duration from non-positive duration in inline-audio exports. Test coverage validates all three changes.

Changes

Audio codec handling and CUDA validation refinements

Layer / File(s)	Summary
Audio codec copy and mux behavior electron/ipc/export/native-video.ts, electron/ipc/export/native-video.test.ts	canCopyAudioCodecIntoMp4 now returns false for empty/unspecified codecs instead of true, triggering transcoding. Tests verify that unknown codecs (WAV) transcode to AAC in MP4 mux operations and that undefined and empty-string inputs are blocked.
CUDA export audio duration validation electron/ipc/export/native-video.ts, electron/ipc/export/native-video.test.ts	validateNvidiaCudaExportSummary now explicitly reports "missing output audio duration" when outputAudioDurationSec is null, and separately flags non-positive duration. Tests verify rejection of inline-audio CUDA output when duration is missing.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• webadderallorg/Recordly#558: Both PRs update validateNvidiaCudaExportSummary to refine CUDA inline-audio validation, with this PR's missing/non-positive duration distinction building directly on the same validator logic.

Suggested labels

Checked

Poem

🐰 A rabbit hops through codec streams,
Splitting truths from missing dreams—
Null is null, and zero's zero,
Validation's now a clearer hero!
Empty codecs find their way,
Transcoded right another day. ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main changes: tightening audio safety validation and codec handling for native video exports.
Description check	✅ Passed	The description covers key sections (Summary, Why, Verification) and explains the problem, solution, and testing approach, though it does not follow the template structure exactly.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/native-audio-safety-guards

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