fix: resolve crashes in DiffStreamHandler — unsafe cast and negative line numbers

[RomneyDa]

Summary

• Fix A: Use safe cast (as?) for TextEditor in undoChanges() to prevent ClassCastException/NullPointerException when the selected editor is not a TextEditor
• Fix B: Add early return for empty documents and clamp line numbers with coerceAtLeast(0) in initUnfinishedRangeHighlights() to prevent IndexOutOfBoundsException
• Fix C: Remove redundant .toList() call in rejectAll() — the list was already copied on the previous line

Fixes #4267

Test plan

• Open a diff stream on an empty file — verify no crash in initUnfinishedRangeHighlights()
• Trigger undoChanges() when the active editor is not a TextEditor (e.g., image viewer) — verify graceful no-op instead of crash
• Reject all diff blocks after accepting/rejecting one — verify behavior unchanged

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Summary by cubic

Fixes two crashes in DiffStreamHandler by safely handling non-TextEditor cases and clamping line numbers for empty or short documents. Prevents crashes when opening diffs on empty files and when undo is triggered in non-text editors. Fixes #4267.

• Bug Fixes

  • Use safe cast in undoChanges() and no-op if not a TextEditor to avoid ClassCastException/NullPointerException.
  • Early return for empty documents and clamp line numbers in initUnfinishedRangeHighlights() to avoid IndexOutOfBoundsException.

• Refactors

  • Remove redundant .toList() in rejectAll() to avoid an extra allocation.
  • Inline lineNum in initUnfinishedRangeHighlights() for clarity.

^{Written for commit 324f1a1. Summary will update on new commits.}

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Docs Review

No documentation updates needed for this PR.

Reasoning: These changes are internal defensive programming fixes to the JetBrains IntelliJ extension's DiffStreamHandler. The PR addresses edge-case crashes (safe casting, bounds checking, code cleanup) without introducing any new features, configuration options, or user-facing behavior changes. The existing documentation remains accurate and complete.

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Test Coverage Review

This PR fixes crash conditions in DiffStreamHandler.kt with defensive guards:

1. Empty document guard - Early return when lineCount == 0
2. Safe cast - as? TextEditor ?: return instead of unsafe cast
3. Negative line bounds - .coerceAtLeast(0) to prevent negative line numbers

Missing Regression Tests

Per the test coverage guidelines, bug fixes should include regression tests that reproduce the original bug condition and verify the fix. Consider adding tests for:

• Empty document case: Verify initUnfinishedRangeHighlights() doesn't crash on empty documents
• Non-TextEditor scenario: Verify undoChanges() gracefully handles when getSelectedEditor() returns a non-TextEditor
• Line number boundary: Verify no negative line numbers are passed to addLineHighlighter() when startLine > lineCount

The IntelliJ test infrastructure (BasePlatformTestCase) is already used in VerticalDiffBlockTest.kt, so similar patterns could be applied here.

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Automated test coverage check

[cubic-dev-ai]

No issues found across 1 file