fix: don't mutate HybridSearchConfig during hybrid search

[ankurjindal (ecomlisters)]

Fixes #337.

asimilarity_search and asimilarity_search_with_score stored the query in hybrid_search_config.fts_query, and __query_collection wrote fetch_top_k into hybrid_search_config.fusion_function_parameters. Both are the caller's objects. Once the first search set fts_query, later searches on the same store used the first query for the keyword half of hybrid search.

Changes

• Pass the search query via kwargs["fts_query"], which __query_collection already reads as a fallback, instead of assigning it to the config. An fts_query set explicitly in HybridSearchConfig still takes precedence.
• Build a copy of fusion_function_parameters with fetch_top_k set, and pass that to the fusion function instead of writing into the user's dict. The effective fetch_top_k is unchanged (it is still the search's k).

Tests
Added to tests/unit_tests/v2/test_async_pg_vectorstore_search.py, covering both asimilarity_search and asimilarity_search_with_score:

• A store-level or per-call config is not mutated by a search.
• Two consecutive searches with different queries each keyword-search their own query.
• An explicit fts_query in the config still takes precedence.

The first two fail on main and pass with this change. make test: 761 passed. make lint is clean.