test: add VectorField RERANK serialization tests for sync and async search

[petyaslavova]

Change summary

This pull request adds serialization coverage for the RERANK attribute on
VectorField, which had no dedicated test. RERANK is a boolean key-value
attribute for HNSW vector fields on disk-backed (Flex / Auto-Tiering)
deployments; the tests verify it flows through the attributes dict as the
string "TRUE"/"FALSE" and produces a well-formed FT.CREATE argument vector.

Changes are test-only (tests/test_search.py, tests/test_asyncio/test_search.py),
so there is no runtime, public-API, or wire-protocol impact. The tests are pure
serialization checks marked fixed_client and need no Redis server; the async
copy is intentionally a plain (non-async) def. Sync/async parity is preserved.

Test coverage

Adds a mirrored test_vector_field_rerank in both suites. Each constructs a
VectorField("v", "HNSW", {...}) with RERANK set to "TRUE" and "FALSE",
then asserts the type/algorithm tokens, the attribute-count token (4 pairs → 8),
and that RERANK and its value appear in field.args. No product code changed.

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Note

Low Risk
Changes are limited to new unit tests with no production code, API, or protocol impact.

Overview
Adds mirrored test_vector_field_rerank coverage in sync and async search suites for the HNSW RERANK index attribute (disk-backed Flex / Auto-Tiering), which had no dedicated test before.

Each test is marked fixed_client and only inspects VectorField(...).args—no Redis server—asserting VECTOR/HNSW, the attribute-count token (4 pairs → 8), and that RERANK appears with "TRUE" or "FALSE" from the attributes dict. No library or runtime code changes.

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