fix(core): stop replace from editing the wrong similar block

[Yash1927]

Summary

This fixes a case where replace could edit the wrong similar block when it had to rely on approximate matching.

Details

• Makes flexible matching fail instead of editing when more than one match is found and allow_multiple is not enabled.
• Makes regex matching follow the same behavior for multiple matches.
• Tightens fuzzy matching so it stays on the intended block instead of drifting to a similarly shaped block with different identifiers.
• Adds regression tests for flexible, regex, and fuzzy matching around similar-block collisions.

Related Issues

Fixes #26975

How to Validate

1. Run:
   npx vitest run packages/core/src/tools/edit.test.ts

2. Run:
   npx prettier --check packages/core/src/tools/edit.ts packages/core/src/tools/edit.test.ts

3. Run:
   npx eslint packages/core/src/tools/edit.ts packages/core/src/tools/edit.test.ts

4. Run:
   npm run typecheck --workspace @google/gemini-cli-core

Expected result:

• the edit tool test suite passes
• formatting, lint, and typecheck pass for the touched files

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • Windows
    • npm run
    • npx
    • Docker
  • MacOS
  • Linux

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses issues where the 'replace' tool could incorrectly modify multiple similar code blocks when only one was intended. By implementing stricter validation and enforcing 'allow_multiple' constraints across all matching strategies, the tool now provides more predictable and safer code editing behavior.

Highlights

• Improved Matching Safety: Updated flexible, regex, and fuzzy matching strategies to prevent accidental edits when multiple matches are found, unless 'allow_multiple' is explicitly enabled.
• Enhanced Fuzzy Matching Precision: Tightened fuzzy matching logic by introducing identifier token validation, ensuring that edits do not drift to similarly shaped blocks with different identifiers.
• Regression Testing: Added comprehensive test cases for flexible, regex, and fuzzy matching to ensure collisions with similar blocks are handled correctly.

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[github-actions]

🛑 Action Required: Evaluation Approval

Steering changes have been detected in this PR. To prevent regressions, a maintainer must approve the evaluation run before this PR can be merged.

Maintainers:

1. Go to the Workflow Run Summary.
2. Click the yellow 'Review deployments' button.
3. Select the 'eval-gate' environment and click 'Approve'.

Once approved, the evaluation results will be posted here automatically.

[gemini-code-assist]

Code Review

This pull request implements stricter controls for the file editing tools by enforcing the allow_multiple parameter across flexible, regex, and fuzzy replacement strategies to prevent unintended multiple edits. It also enhances the fuzzy matching algorithm by introducing a 'protected tokens' mechanism to ensure that identifiers, such as function names, are preserved during matching. Feedback was provided regarding the trailing newline logic in flexible replacements, which was identified as too aggressive and potentially problematic for deletions. Additionally, a reminder was given to trim string parameters before checking for emptiness to comply with repository guidelines.

[gemini-code-assist]

The current logic for adding a trailing newline to the replacement text is too aggressive. It causes issues with deletions and existing newlines. Additionally, per repository rules, when validating string parameters from tools, ensure the string is trimmed before checking for emptiness to prevent whitespace-only values from being accepted.

      let replacementText = newBlockWithIndent.join('\n');
      if (
        normalizedReplace.trim() !== '' &&
        !replacementText.endsWith('\n') &&
        sourceLines[index + searchLinesStripped.length - 1].endsWith('\n')
      ) {
        replacementText += '\n';
      }

References

1. When validating string parameters from tools, trim the string first and then check for emptiness to prevent whitespace-only values from being accepted.