Add pathseer filter strategy for HNSW vector index

[rlmanrique]

Motivation

Weaviate core is adding pathseer (weaviate/weaviate#12137, targeting 1.40), a third HNSW filter strategy that adaptively decides per query between sweeping and multi-hop candidate discovery. The client needs to expose it so users can configure it — but more importantly, without the enum value the client cannot even read the config of a collection that uses it: collection.config.get() deserializes the server response via VectorFilterStrategy(config["filterStrategy"]) and raises a ValueError on the unknown string. This makes the change a forward-compatibility fix as well as a feature enablement.

Approach

Add PATHSEER = "pathseer" to the existing VectorFilterStrategy enum in config_vector_index.py. No serialization or validation changes are needed — the strategy is passed through as a string and validated server-side, which keeps the client agnostic to strategy semantics (same pattern as ACORN).

Risks and mitigations

• Older servers rejecting the value: the client doesn't gate on server version by design; a pre-1.40 server returns a clear validation error. Consistent with how other newer config options are handled.
• Otherwise trivial: additive enum value, no behavior change for existing strategies.

Testing

• Unit: Configure.VectorIndex.hnsw(filter_strategy=PATHSEER) serializes to "filterStrategy": "pathseer".
• Integration (skipped below server 1.40.0): creates a collection with pathseer, reads it back via config.get() (exercising the deserialization path that previously would have raised), then round-trips Reconfigure updates pathseer → acorn → pathseer to verify the strategy is mutable in both directions. Verified passing against a local 1.40.0-dev server built from the core branch.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca