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 (evaluation,
membership caching, status polling, error handling) against the C++ server SDK.
Builds on #552.

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

• The store does each lookup on its own short-lived io_context rather than the
  SDK executor, so a synchronous callback can't deadlock the SDK's single io
  thread during a metadata poll. A non-200 from the harness maps to a store
  error; lastUpToDate of 0 maps to "never synchronized".
• 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.