fix: enforce mandatory :start_line: in apply_diff tool

[roomote-v0]

Related GitHub Issue

Closes: #12199

Description

This PR attempts to address Issue #12199 by enforcing the :start_line: directive as mandatory in the apply_diff tool, matching what the tool prompt already documents.

Changes:

1. src/core/diff/strategies/multi-search-replace.ts:

   • Made :start_line: mandatory in the regex pattern (removed trailing ? from the group)
   • Added :start_line: presence tracking (foundStartLine) in validateMarkerSequencing
   • Updated capture group indices to reflect the new regex structure
   • Updated error messages to reflect the requirement

2. src/core/tools/ApplyDiffTool.ts:

   • Added early validation that :start_line: is present before calling applyDiff

3. src/core/task/Task.ts:

   • Added sayAndCreateMissingStartLineError helper method following the existing sayAndCreateMissingParamError pattern

4. Test files:

   • Updated all existing tests to include :start_line: directive
   • Added a new test case verifying that diffs without :start_line: are rejected

Test Procedure

• All 64 tests in multi-search-replace.spec.ts and multi-search-replace-trailing-newline.spec.ts pass
• Lint and type-check pass across the full monorepo
• New test added: "rejects diff without :start_line:" validates the new enforcement

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue.
• Scope: Changes are focused on the linked issue.
• Self-Review: Performed a thorough self-review.
• Testing: Updated and added tests to cover changes.
• Documentation Impact: No documentation changes needed (the tool prompt already documents :start_line: as required).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Feedback and guidance are welcome.

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