feat: Add C bindings for Redis Big Segments store

[beekld]

Summary

Adds a C binding for RedisBigSegmentStore so C callers can wire the existing Redis Big Segments integration into the SDK's Big Segments config.

New public surface:

• LDServerBigSegmentsRedisStore opaque handle.
• struct LDServerBigSegmentsRedisResult with store and error_message fields.
• LDSERVER_BIGSEGMENTS_REDISSTORE_ERROR_MESSAGE_SIZE macro.
• LDServerBigSegmentsRedisStore_New(uri, prefix, out_result) factory.
• LDServerBigSegmentsRedisStore_Free(store).

Tests:

• Unit tests for successful creation, failure error propagation, and null-store on failure.
• End-to-end test against a live Redis: primes a sync timestamp, wires the store through the C API, and verifies the SDK reports the store available via the Big Segment status listener.

Design decision worth flagging: the type name uses config-section-first word order (BigSegments + Redis + Store) to mirror the existing Redis LazyLoad C binding's LDServerLazyLoadRedisSource (LazyLoad + Redis + Source). This differs from the C++ class name RedisBigSegmentStore -- a literal C++ mirror would have been LDServerRedisBigSegmentStore. Chose consistency with the sibling C binding in the same header directory over C++ class-name parity.

Stacked on #573 (Big Segments C bindings). Please review that PR first.

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Note

Low Risk
New public C API and thin wrapper around existing C++ store logic; no changes to core flag evaluation or auth paths.

Overview
Exposes a C API for the existing Redis-backed Big Segments store so C callers can plug it into LDServerBigSegmentsBuilder the same way the Lazy Load Redis binding works for data sources.

The new surface includes an opaque LDServerBigSegmentsRedisStore handle, a stack-allocated LDServerBigSegmentsRedisResult (store pointer plus fixed-size error_message), and LDServerBigSegmentsRedisStore_New / _Free. The implementation delegates to RedisBigSegmentStore::Create and documents that the SDK may take ownership instead of calling _Free. The redis-source library build now compiles bindings/redis/redis_big_segment_store.cpp.

Tests cover create/free and failure paths (non-empty error, null store), plus an integration test that seeds Redis sync metadata, wires the store through the C Big Segments config, and waits on the Big Segment store status listener until available.

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