Fix multiline partial-line suffix preservation

[szgupta]

Description

Fix diff validation for search/replace edits whose final search line is a partial prefix of the matched file line when the replacement deletes earlier lines.

Previously suffix preservation only ran when the search and replacement had the same line count. That missed cases where the model kept a partial trailing context line, but removed middle lines, causing the unchanged suffix of the final matched file line to be dropped. This adds a helper that preserves the final-line suffix when the replacement's final line carries the same partial context, and keeps the existing same-line-count behavior.

Linked Issue

N/A — follow-up regression from Agent Mode diff application behavior.

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Screenshots / Videos

N/A — diff validation logic change.

Testing

• cargo fmt
• cargo clippy --workspace --exclude warp_completer --all-targets --tests -- -D warnings
• cargo clippy -p warp_completer --all-targets --tests -- -D warnings
• cargo nextest run -p ai -E 'test(diff_validation)'

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Oz conversation

CHANGELOG-BUG-FIX: Fixed file edit diffs dropping the end of a partial trailing context line when applying multi-line replacements.

Co-Authored-By: Oz oz-agent@warp.dev

[oz-for-oss]

@szgupta

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[oz-for-oss]

Overview

This PR refactors partial-line suffix preservation into a helper and extends it to multi-line replacements that delete earlier lines while retaining the final partial context line. It also adds a regression test covering the dropped-suffix case.

Concerns

• No blocking correctness concerns found in the changed lines.
• Security pass: no security-sensitive behavior or new trust boundary was introduced by this string-preservation logic.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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