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.

---

Note

Low Risk
Changes are confined to contract-test infrastructure and data-model JSON types; production SDK behavior is unchanged.

Overview
Adds Big Segments coverage to the C++ server contract-test harness so the shared suite can drive the server SDK end-to-end.

A new ContractTestBigSegmentStore implements IBigSegmentStore by synchronously POSTing to harness callback paths (/getMembership, /getMetadata), similar in spirit to ContractTestHook but with request/response semantics required by the store API.

The contract data model gains ConfigBigSegmentsParams on ConfigParams, EntityManager wires it through BigSegmentsBuilder (cache, poll interval, stale-after), and ClientEntity handles the new getBigSegmentStoreStatus command via Client::BigSegmentStoreStatus(). The service advertises the big-segments capability.

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