fix(apply-diff): reject partially matched stacked anchors - #4431

Summary

This pull request fixes stacked @@ anchors so every non-empty anchor in a stack must match in sequence. Previously, if one anchor matched and a later anchor did not, apply_diff continued from the partial match and could apply the hunk body to unrelated nearby content.

Single unmatched anchors keep their existing context fallback. Bare anchors, whitespace-fuzzy matches, and fully matched stacks also keep their existing behavior. A partially matched stack now raises ValueError before edited content is returned; the sandbox boundary converts that error to ApplyPatchDiffError before writing the file.

This brings the Python parser in line with the same safety rule added in openai/openai-agents-js#1655.

Test plan

• Added direct parser coverage for partial stacks, missing first anchors, missing anchors followed by bare markers, and single-anchor compatibility.
• Added sandbox coverage proving a rejected partial stack leaves the original file unchanged.
• Ran .agents/skills/code-change-verification/scripts/run.sh with format, lint, typecheck, and the complete test suite passing.

Issue number

N/A. Found while comparing stacked-anchor behavior across the Python and JavaScript SDKs.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR