Test subtree-to-inline storage conversion in multimap removal

[cberner]

Summary

Extends the multimap_remove_subtree_backed_key test to verify that B-tree subtree storage is correctly converted back to inline storage when removals shrink the data below the threshold.

Changes

• Enhanced test coverage: Added a new test section that removes almost all values from a key that was previously stored in subtree (B-tree) format, verifying the storage conversion path
• Updated test documentation: Clarified the test's purpose to include coverage of the subtree-to-inline shrinking behavior
• Verification assertions: Added checks to confirm:
  • The subtree correctly shrinks to a single remaining value
  • The total table length is correct after removals
  • The remaining value is correctly accessible after the storage conversion

Implementation Details

The new test section removes 998 out of 999 values for key 0 (keeping only value 500), which causes the B-tree subtree to shrink below the page-size threshold and triggers automatic conversion back to inline storage. This ensures the multimap implementation correctly handles the transition between storage formats during the removal process.

https://claude.ai/code/session_01FPYFdGyznM7nMorDiAK5A2

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.50%. Comparing base (25d2db0) to head (992b256).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1256      +/-   ##
==========================================
+ Coverage   88.34%   88.50%   +0.16%     
==========================================
  Files          36       36              
  Lines       14848    14848              
==========================================
+ Hits        13117    13141      +24     
+ Misses       1731     1707      -24     

see 2 files with indirect coverage changes

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