test: add e2e regression tests for issue #1138 — AI insert between existing lines

[svarlamov]

Summary

Adds three e2e tests using the TestRepo harness with custom checkpoints to verify whether #1138 still reproduces.

Finding: The bug reproduces. When AI inserts content at multiple positions in a single edit, the line ranges for later insertion blocks are truncated, causing the tail lines to be misattributed as unknown/untracked instead of AI.

Tests added

Test	Result	Description
test_ai_insert_between_existing_methods_issue_1138	✅ Pass	Single AI method block inserted between two existing methods. All lines correctly AI-attributed.
test_ai_insert_single_line_between_existing_lines_issue_1138	✅ Pass	Single AI line inserted between existing lines. Correctly attributed.
test_ai_insert_multiple_blocks_between_existing_issue_1138	❌ Fail (ignored)	Two AI method blocks inserted at different positions in a single edit. Lines 19-20 of the second block are misattributed as Test User instead of mock_ai, confirming the lineRanges bug.

Blame output from the failing test

Lines 16-18: mock_ai     ← correct (first 3 lines of second AI block)
Line  19:    Test User   ← WRONG (closing brace of helperBC)
Line  20:    Test User   ← WRONG (trailing blank line)

The third test is marked #[ignore] so CI passes. Remove it once the underlying line-range calculation is fixed.

Review & Testing Checklist for Human

• Run the ignored test to confirm the reproduction: task test TEST_FILTER=test_ai_insert_multiple_blocks EXTRA_TEST_BINARY_ARGS="--ignored" NO_CAPTURE=true
• Verify the two passing tests cover the expected single-block insertion scenarios

Notes

• All three tests use explicit fs::write + manual checkpoint human / checkpoint mock_ai calls (not set_contents) to precisely replicate real AI agent checkpoint flows.
• The bug appears specific to multi-position insertions in a single edit — single-block insertions work correctly.

Link to Devin session: https://app.devin.ai/sessions/c78741bb99c14c0ca7ce1536292df196
Requested by: @svarlamov

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.