feat: Add Big Segments support to the server contract-test service

[beekld]

Add Big Segments support to the server contract-test service

Lets the shared contract-test harness run its Big Segments suite against the C++ server SDK.

What's in

• ContractTestBigSegmentStore: an IBigSegmentStore that delegates
  getMembership/getMetadata to the harness over HTTP (synchronous
  request/response, mirroring ContractTestHook's callback wiring).
• A bigSegments config block in the contract-tests data model ->
  ConfigBuilder::BigSegments(...) in the entity manager.
• The getBigSegmentStoreStatus command -> Client::BigSegmentStoreStatus().
• big-segments added to the advertised capabilities.

No SDK changes here -- bigSegmentsStatus already serializes into the evaluation
reason, so it flows through the existing evaluate path.

Design notes

• This mirrors how the Java SDK covers Big Segments end-to-end. The store
  implementations themselves are tested against real Redis/DynamoDB at the
  source-library level separately.

Testing

• The harness Big Segments suite passes against the built service (40 ran,
  13 skipped, 0 failed).
• server-tests builds and advertises big-segments.
• Full server-sdk unit suite green.

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Note

Low Risk
Changes are confined to contract-test harness code and JSON protocol types; production SDK behavior is unchanged aside from using existing Big Segments APIs.

Overview
Enables the shared contract-test harness to run its Big Segments suite against the C++ server SDK by wiring harness-driven store callbacks into client configuration and exposing store health via the existing command protocol.

The contract-tests data model gains ConfigBigSegmentsParams (callback URI plus optional cache/poll/stale tuning) on ConfigParams, a BigSegmentStoreStatusResponse type, and a new getBigSegmentStoreStatus command. ContractTestBigSegmentStore implements IBigSegmentStore with synchronous HTTP POSTs to the harness for /getMembership and /getMetadata, analogous to hook callbacks but blocking because lookups must return values. EntityManager maps the bigSegments config block to BigSegmentsBuilder and the shared store instance; ClientEntity handles the new command via Client::BigSegmentStoreStatus(). The service advertises the big-segments capability and builds the new store source into server-tests.

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