Important

Disallow empty search content (insertions) in MultiSearchReplaceDiffStrategy, remove related tests, and add a fuzzy matching test for issue #2556.

• Behavior change in multi-search-replace.ts:
  • Disallows empty search content in MultiSearchReplaceDiffStrategy.applyDiff(). Now returns an error if search content is empty, removing support for insertions via empty search blocks.
  • Updates getSimilarity() to return 0 for empty search content (was 1).
• Tests in multi-search-replace.test.ts:
  • Removes all tests for insertions (empty search block with non-empty replace block).
  • Adds test case for fuzzy matching with slightly off line numbers to reproduce Incorrect applying of apply_diff #2556.
  • Renames test group from "insertion/deletion" to "deletion" to reflect removal of insertion tests.
• No changes to deletion or replacement logic.

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