fix: enhance null edit detection for Gemini models

[roomote]

Summary

This PR addresses Issue #7360 where Gemini models (2.5 Pro, 2.5 Flash, 2.5 Flash:Thinking) produce "phantom" edits that appear to make changes but actually result in zero character modifications.

Problem

The user reported that Gemini models would get into non-productive loops where they would claim to be making edits but the search and replace blocks would contain identical content, resulting in no actual changes to the files.

Solution

Enhanced the diff validation in both diff strategies to:

1. Detect when search and replace content are identical (null edits)
2. Provide clear, detailed error messages specifically mentioning this is a common Gemini issue
3. Guide users with actionable steps to fix the problem
4. List common causes (model hallucination, copy/paste errors, missing modifications)

Changes

• Enhanced error messages in multi-search-replace.ts and multi-file-search-replace.ts
• Added comprehensive test coverage in null-edit-detection.spec.ts
• Tests verify null edit detection while ensuring legitimate deletions still work

Testing

• ✅ All new tests passing (5/5)
• ✅ No regression in existing tests (60/60 passing)
• ✅ Linting and type checking pass

Impact

This change will help AI models, especially Gemini, avoid producing phantom edits and provide clear guidance when null edits are detected, improving the overall user experience.

Fixes #7360

---

Important

Enhances null edit detection for Gemini models by improving error messages and adding comprehensive test coverage.

• Behavior:
  • Enhanced null edit detection in multi-search-replace.ts and multi-file-search-replace.ts to identify identical search and replace content.
  • Provides detailed error messages for null edits, highlighting common causes and solutions.
• Testing:
  • Added null-edit-detection.spec.ts to test null edit detection, ensuring legitimate deletions are not flagged.
  • Tests cover scenarios like identical search/replace, multi-block diffs, and line number handling.
• Impact:
  • Improves user experience by preventing non-productive loops in Gemini models and guiding users to resolve null edits.

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[roomote]

Reviewing my own code is like debugging in a mirror - everything looks backwards but the bugs are still mine.

[roomote]

Consider extracting this error message to a shared constant since it's duplicated in multi-file-search-replace.ts (lines 518-528). This would ensure consistency and make future updates easier.

Also, while the error message is comprehensive, we might want to add a test case for when search and replace blocks differ only in whitespace type (tabs vs spaces) to ensure we're not triggering false positives.

[roomote]

Great test coverage! Consider adding a comment here explaining why legitimate deletions (empty replace blocks) should not trigger null edit detection. This distinction is important for future maintainers to understand the design decision.

[roomote]

Could we add an edge case test for when search and replace blocks have different whitespace but are otherwise identical? This would help document the expected behavior when whitespace types differ (tabs vs spaces).

[bstrdsmkr]

Just adding another data point - I'm seeing this on self-hosted llama-4-maverick too, so likely not model or provider specific

[daniel-lxs]

#7360 (comment)