feat: expose Big Segments via public Client and Config APIs

[beekld]

Expose Big Segments to customer code

Makes the server-side Big Segments feature reachable from customer code: a public config method to wire a store, and a status provider on the client for querying store health and subscribing to changes. Builds on SDK-2366-2.

What's in

• Promotes BigSegmentsBuilder to the public include tree and adds ConfigBuilder::BigSegments(BigSegmentsBuilder), surfaced as an optional Config::BigSegments().
• New public IBigSegmentStoreStatusProvider and BigSegmentStoreStatus (IsAvailable()/IsStale()), plus Client::BigSegmentStoreStatus().
• ClientImpl constructs the BigSegmentStoreWrapper from config, hands it to the evaluator, and starts its background poll. An internal adapter exposes the wrapper's status broadcaster as the public provider.
• BigSegmentsBuilder::Build() now returns a new error code when the store is null.

Design notes

• The status surface mirrors DataSourceStatus: the accessor is Status(), and listeners use OnBigSegmentStoreStatusChange(handler) -> IConnection.
• When no store is configured, the provider reports the store as unavailable and registered listeners never fire.

Testing

Unit tests added for the new behavior.

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Note

Medium Risk
Changes flag evaluation wiring and add a new public config surface; misconfiguration or store outages can affect Big Segment targeting, though behavior is gated on optional config and mirrors existing data-source status patterns.

Overview
This PR wires Big Segments into the public server SDK: customers can attach a store via config and observe store health on the client, similar to data source status.

Configuration: ConfigBuilder::BigSegments(BigSegmentsBuilder) stores an optional BigSegmentsConfig on Config. BigSegmentsBuilder::Build() now returns tl::expected and fails with kConfig_BigSegments_NullStore when the store pointer is null (previously a null store was allowed through).

Client runtime: When Big Segments are configured, ClientImpl creates BigSegmentStoreWrapper, passes it to the evaluator, starts background metadata polling, and exposes status through Client::BigSegmentStoreStatus().

Public status API: New BigSegmentStoreStatus (IsAvailable / IsStale) and IBigSegmentStoreStatusProvider with Status() and OnBigSegmentStoreStatusChange → IConnection. An internal adapter maps the wrapper to this API; with no store configured, status is unavailable/not stale and listeners are no-ops.

Unit tests cover builder validation, config round-trip, and status provider behavior (including transitions).

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[kinyoklion]

Do we need some exception wrapping around the handler?

[beekld]

None of the other public APIs that take callbacks seem to wrap the handlers, so I don't think so. We could do a pass and add it to all of them if we think it's a good idea.