fix(vamana): honor asymmetric query metrics

[luoxiaojian]

No description provided.

[richyreachy]

LGTM

[Copilot]

Pull request overview

This PR fixes Vamana streamer behavior for asymmetric metrics by ensuring build-time (add) distance functions and query-time (search) distance functions can differ, so searches honor the metric’s query_metric() when provided.

Changes:

• Cache separate distance/batch-distance callables for add vs search in VamanaStreamer::open().
• Update VamanaContext/VamanaDistCalculator to allow swapping distance implementations per operation.
• Add a regression test validating that brute-force and graph search use the asymmetric query metric while add/build uses the base metric.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/core/algorithm/vamana/vamana_streamer_test.cc	Adds regression test covering asymmetric query metric behavior across add, BF search, graph search, and BF-by-primary-keys search.
src/core/algorithm/vamana/vamana_streamer.h	Introduces cached add/search distance and batch-distance function members.
src/core/algorithm/vamana/vamana_streamer.cc	Initializes add/search distance callables in open() and applies them to contexts in add/search code paths.
src/core/algorithm/vamana/vamana_dist_calculator.h	Adds a method to update the distance and batch-distance callables dynamically.
src/core/algorithm/vamana/vamana_context.h	Exposes a context-level helper to update the underlying distance calculator’s callables.

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