fix(ext): reject stale hunks in TextCanvas.apply_patch with context validation

[Manohar Paturi (ManoharPaturi)]

Fixes #8193

Summary

TextCanvas.apply_patch() documented context-line validation, but the hunk loop performed a blind working_lines[start:end] = replacement slice assignment — so a unified diff prepared against an older revision was silently applied to the latest revision, returning PATCH APPLIED and making stale content the newest revision (reproduced exactly as in the issue: stale edit landed, region=Tokyo).

Changes

• Each hunk's expected source block (removed + context lines, in order) is now built alongside the replacement and compared exactly against the current content slice. Mismatch or out-of-bounds hunks raise ValueError, matching the class's existing error convention and propagating through ApplyPatchTool like its other validation errors. The check runs before add_or_update_file(), so rejected patches leave the canvas untouched.
• Related correctness fix required by the above: the insertion index for zero-length source hunks is now source_start (not source_start - 1), which fixes silent one-line misplacement of n=0 difflib patches while keeping file-creation patches working.

Tests

python/packages/autogen-ext/tests/memory/test_text_canvas_memory.py (+100):

• test_apply_patch_rejects_stale_hunks — the issue's repro now raises ValueError, revision 2 stays latest, and a control patch built from the real revision-2 content then applies cleanly (revision 3)
• test_apply_patch_rejects_near_miss_context — one-char context drift is rejected (no fuzzy matching)
• valid-patch coverage via the pre-existing test_apply_patch_increments_revision

pytest tests/memory/test_text_canvas_memory.py → 6/6 passed; ruff check + ruff format --check clean on both files. Multi-hunk, deletions, EOF appends, prepends, create-empty, zero-context insert, and chained applies manually verified unaffected.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.