fix: show complete diff previews

[chryzxc]

Summary

• remove the fixed 40-line diff preview truncation
• preserve complete multi-hunk diffs with scrollable expanded previews
• reset line numbering for each hunk
• add regression coverage for the issue

Validation

• npm run verify
• diff-preview regression tests
• TypeScript typecheck

Closes #12.

Summary by CodeRabbit

• Bug Fixes
  • Diff previews now display all available patch content, including later hunks, without truncating the preview.
  • Line numbers reset correctly for each diff hunk.
  • Duplicate hunk headers are no longer shown in diff excerpts.
  • Added regression coverage to help prevent these rendering issues from returning.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 63d02ebc-8280-4ae0-9ab1-061c3170082f

📥 Commits

Reviewing files that changed from the base of the PR and between 0b6fa54 and 95a5de9.

📒 Files selected for processing (3)

• tests/regression/diff-preview-block-grouping-regression.test.mjs
• webview/shared/src/chat/components/ActivityDiffExcerpt.tsx
• webview/shared/src/chat/components/activity-steps/DiffPreviewStep.tsx

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📝 Walkthrough

Walkthrough

Diff preview parsing now retains complete patches instead of truncating after 40 lines. Excerpt rendering recognizes additional hunk headers, resets line numbering per hunk, avoids duplicate headers, and adds regression tests for both behaviors.

Changes

Diff preview rendering

Layer / File(s)	Summary
Complete diff excerpt construction webview/shared/src/chat/components/activity-steps/DiffPreviewStep.tsx	The parser selects the first filtered hunk header, preserves all diff lines, and calculates addition/deletion counts from the full patch.
Multi-hunk line numbering webview/shared/src/chat/components/ActivityDiffExcerpt.tsx	Line numbering resets for each embedded hunk header, which is marked as a header row; duplicated initial headers are skipped.
Regression coverage tests/regression/diff-preview-block-grouping-regression.test.mjs	Tests verify complete patch retention and hunk-based line counter resets.

Estimated code review effort: 2 (Simple) | ~15 minutes

✨ 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 agent/fix-diff-preview-issue-12

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