fix(v2): don't mutate instance hybrid_search_config in similarity searches

[Humphrey (HumphreySun98)]

Description

`AsyncPGVectorStore.asimilarity_search` and `asimilarity_search_with_score` back-fill `fts_query` on the `HybridSearchConfig` object retrieved from kwargs or, by default, from `self.hybrid_search_config`:

```python
hybrid_search_config = kwargs.get("hybrid_search_config", self.hybrid_search_config)
if hybrid_search_config and not hybrid_search_config.fts_query:
hybrid_search_config.fts_query = query # ← in-place
kwargs["hybrid_search_config"] = hybrid_search_config
```

Because the assignment hits the shared object in place, the very first call's `query` sticks on it and every subsequent search reuses that `fts_query` regardless of the new `query` argument. Same hazard for callers that pass the same `HybridSearchConfig` to multiple searches. This is exactly the symptom reported in #288.

The fix is to copy the config before back-filling, mirroring how other LangChain integrations handle caller-owned input (see e.g. langchain-anthropic's recent `bind_tools` mutation fix).

Relevant issues

Fixes #288

Changes

• `langchain_postgres/v2/async_vectorstore.py`: in both `asimilarity_search` and `asimilarity_search_with_score`, `copy.copy(hybrid_search_config)` before assigning `fts_query`. The `copy` module is already imported at the top of the file.
• `tests/unit_tests/v2/test_async_vectorstore_no_mutation.py`: four async unit tests that bypass `AsyncPGVectorStore.create(...)` (no Postgres needed) and pin:
  • instance-level config is preserved across two calls;
  • caller-provided config is preserved when reused across calls;
  • existing `fts_query` is not overwritten (back-fill branch correctly skips);
  • `asimilarity_search_with_score` has the same invariants.

Testing

```
$ python -m pytest tests/unit_tests/v2/test_async_vectorstore_no_mutation.py -q
4 passed in 0.49s
```

Reverting only the `copy.copy(...)` line while keeping the new tests makes three of the four tests fail (the fourth covers the no-back-fill branch and is unaffected), confirming the regression coverage pins the buggy behavior. `ruff check` and `ruff format --check` pass on both files.

Note

Fix is intentionally minimal — a shallow `copy.copy` is sufficient because `HybridSearchConfig` is a flat dataclass and only `fts_query` (a string) is reassigned. No callers need updating; non-hybrid-search paths are untouched.

Disclaimer: this PR was prepared with the assistance of an AI agent (Claude Code). All code and test changes were reviewed by the author before submission.