Fix flaky SVS tiered flow test by removing incorrect flat buffer assertion

[meiravgri]

Remove an incorrect assertion in search_insert that checked get_curr_bf_size() == 0 immediately after wait_for_index(1).

The flat buffer is not guaranteed to be empty at that point: vectors added during the current update job — but not enough to trigger another one — may still reside in the buffer. This caused flaky test failures.

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.99%. Comparing base (d7a5afb) to head (c61d002).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #934   +/-   ##
=======================================
  Coverage   96.99%   96.99%           
=======================================
  Files         129      129           
  Lines        7576     7576           
=======================================
  Hits         7348     7348           
  Misses        228      228           

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