fix(v2): make ExactNearestNeighbor instantiable (unblocks exact search)

[Andrew Chen (chuenchen309)]

Problem

ExactNearestNeighbor is the intended way to switch a PGVectorStore to
brute-force / exact search — apply_vector_index treats it specially and drops
the ANN index for it. But the class cannot be constructed:

@dataclass
class ExactNearestNeighbor(BaseIndex):
    index_type: str = "exactnearestneighbor"

BaseIndex declares index_options as an @abstractmethod, and
ExactNearestNeighbor never implements it, so it stays abstract:

>>> from langchain_postgres.v2.indexes import ExactNearestNeighbor
>>> ExactNearestNeighbor()
TypeError: Can't instantiate abstract class ExactNearestNeighbor without an
implementation for abstract method 'index_options'

Because no instance can ever be created, the exact-search branch in
aapply_vector_index is dead code:

async def aapply_vector_index(self, index: BaseIndex, ...):
    if isinstance(index, ExactNearestNeighbor):
        await self.adrop_vector_index()
        return
    ...
    params = "WITH " + index.index_options()

HNSWIndex / IVFFlatIndex work because they implement index_options.

Fix

Implement index_options on ExactNearestNeighbor returning an empty string.
This is safe: aapply_vector_index short-circuits (drops the index and returns)
before index_options() is ever used to build DDL, so the returned value is
never consumed — it exists only to make the class concrete/instantiable.

Test

Added test_exact_nearest_neighbor_index to
tests/unit_tests/v2/test_indexes.py (pure-Python, no live Postgres): it
constructs ExactNearestNeighbor() and checks index_type/index_options().
Fails before the change (TypeError), passes after.

$ python -m pytest tests/unit_tests/v2/test_indexes.py -q
6 passed

ruff check passes on the changed files.

---

Disclosure: I used AI assistance (Claude) to help find this
uninstantiable-class / dead-branch issue and write the test. I reviewed the
change, ran the tests, and take responsibility for its correctness.