Fix #10193: harden SoftNI regression tests (review follow-up)

[Ironship]

Problem

Follow-up to merged PR #13241 (issue #10193, SoftNI sub export frame rate). Copilot review on #13241 noted that the regression tests index lines[timingIndex + 1] right after asserting the TIMING header exists, without checking that a following line actually exists — if the output is truncated or TIMING is the last line, the test would throw IndexOutOfRangeException instead of failing with a clear assertion message. The comment applies at two sites (line 31 and line 51 of tests/libse/SubtitleFormats/SoftNiSubTest.cs).

Fix

Added a bounds assertion before indexing in both SoftNI tests:
Assert.True(timingIndex + 1 < lines.Count, "Missing timing line after *TIMING* section");

Test-only change; no production code touched.

Verification

• dotnet build tests/libse/LibSETests.csproj — EXIT:0, 0 errors
• dotnet test tests/libse/LibSETests.csproj --filter FullyQualifiedName~SoftNiSubTest — Passed: 4, Failed: 0
• Diff limited to 2 added assertion lines

Notes

• Copilot review finding from PR Fix #10193: SoftNI sub export writes project frame rate instead of hardcoded 25 fps #13241 addressed here as a follow-up PR since Fix #10193: SoftNI sub export writes project frame rate instead of hardcoded 25 fps #13241 was already merged by the maintainer.
• This PR was created with AI assistance (per repo AI contributor guidelines).

[Ironship]

/copilot-review

[Copilot]

🟢 Ready to approve

The change is a small, test-only hardening that prevents an out-of-range crash and aligns exactly with the stated follow-up goal.

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Pull request overview

This PR hardens the SoftNI regression tests added for issue #10193 by ensuring the tests fail with a clear assertion message (instead of throwing IndexOutOfRangeException) if the exported output is truncated or *TIMING* is the last line.

Changes:

• Add a bounds assertion (timingIndex + 1 < lines.Count) before indexing the timing line in SoftNiSub test.
• Add the same bounds assertion in the SoftNiColonSub test.

File summaries

File	Description
tests/libse/SubtitleFormats/SoftNiSubTest.cs	Adds explicit bounds assertions before indexing the line following *TIMING*, improving test failure clarity and robustness.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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[Ironship]

CI note: the test job failure is a pre-existing Avalonia headless flake, not a regression from this PR:

• Failing test: UITests.Features.Shared.TextBoxTagTogglerTests.ToggleTag_SelectedItalicWord_RemovesItalic — a Test Case Cleanup Failure (System.InvalidOperationException: The calling thread cannot access this object because a different thread owns it), i.e. thread-affinity in teardown.
• The test is unrelated to this PR (test-only hardening of SoftNiSubTest); LibSE tests all passed (859/859, incl. the 4 SoftNI tests).
• The same flake class hits a different test on every full-suite run (locally: SubtitleGridScrollPerformanceTests.HomeAndEnd_RealizeOnlyAViewportOfRows failed in two consecutive full runs while passing in isolation; here CI failed TextBoxTagTogglerTests, which passes 5/5 in isolation on this exact branch).
• Different test each run + teardown thread-affinity exception + passes in isolation = infrastructure flake, not a code issue.