fix: KeyError 'pq' when updating the quantizer on an HFresh vector index

[Shah91n]

You can't change the RQ settings on an HFresh index. Trying to update rq.rescore_limit fails straight away with KeyError: 'pq', before the client even talks to the server. The docs say this setting can be changed at runtime, and the server does accept it if you edit the schema directly, so this is a client-side bug.

The cause is in the check that stops you from switching quantizers. When you ask to update RQ, the client first looks at whether PQ, BQ or SQ are currently switched on, since swapping quantizer types means recreating the collection. That check assumes every index has all four quantizer blocks in its schema, which is true for HNSW. HFresh always uses RQ, so its schema only has an rq block, and looking up pq throws.

Adds tests covering the HFresh case, which fail without the fix, plus one pinning that switching quantizers on HNSW is still rejected. Verified against Weaviate 1.38.7, where rq.rescore_limit now updates as expected.

[orca-security-eu]

Orca Security Scan Summary

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[Copilot]

Pull request overview

Fixes a client-side KeyError: 'pq' raised when updating RQ quantizer settings on an HFresh vector index by making the quantizer-switch guard tolerant of schemas that don’t include pq (HFresh schemas may only contain an rq block). This aligns client behavior with server behavior and documented runtime mutability of rq.rescoreLimit.

Changes:

• Avoid direct subscripting of vector_index_config["pq"] in the quantizer-switch guard; use a safe default when pq is missing.
• Add regression tests covering updating RQ on HFresh schemas without pq/bq/sq blocks.
• Add a test ensuring quantizer switching remains rejected on HNSW when PQ is enabled.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
weaviate/collections/classes/config.py	Makes quantizer-switch guard robust to missing pq block in vector index config (prevents KeyError).
test/collection/test_config_update.py	Adds regression tests for HFresh RQ updates without a pq block and keeps HNSW quantizer-switch rejection covered.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.54%. Comparing base (95b5d76) to head (5c8f7a5).
⚠️ Report is 79 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2147      +/-   ##
==========================================
+ Coverage   86.64%   88.54%   +1.89%     
==========================================
  Files         300      304       +4     
  Lines       23172    23486     +314     
==========================================
+ Hits        20077    20795     +718     
+ Misses       3095     2691     -404     

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