feat: Add the FDv2 data source orchestrator to the client SDK

[beekld]

Summary

Adds the client SDK's FDv2 data source -- the orchestrator that runs initializers to load a basis, then rotates synchronizers to keep it current.

Implemented here:

• Runs initializers to a basis, then rotates synchronizers on fallback and recovery conditions.
• Reports data-source status, records the environment id, and tags applied data as cache- or network-sourced.

Also moves the shared orchestration primitives out of the server SDK into libs/internal so both SDKs can share them. That server change is a behavior-neutral relocation.

Notes:

• Start() blocks on cache initialization, so cached flags are immediately evaluable. This matches the behavior of the FDv1 client. It also matches Android. However, Flutter loads the cache asynchronously instead.
• FDv1 fallback directive handling is out of scope.

---

Note

Overview
Introduces FDv2DataSource for the client SDK: an IDataSource that chains initializers (until a selector-bearing basis exists), then synchronizers with fallback/recovery conditions, applies changesets to the flag store (with from_cache tagging), publishes data-source status, and exposes EnvironmentId(). Start() synchronously runs cache initializers so cached flags are evaluable before the executor runs, including cache-only / offline success paths.

Refactors shared FDv2 orchestration into libs/internal (conditions, SourceManager, condition interfaces) under launchdarkly::internal::data_sources, so client and server both consume the same code. Conditions now take a SourceSignal (kChangeSet / kInterrupted / kOther) instead of full source results; orchestrators map results via ClassifyResult. SourceManager becomes a header-only template; the server SDK drops its local copies and source_manager.cpp.

Adds broad unit tests for the client orchestrator and updates internal/server tests for the new APIs.

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[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

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[cursor]

Shutdown does not fence updates

High Severity

Close() does not stop in-flight work from writing flags or status. ApplyResult never consults closed_, OnInitializerResult applies a changeset before checking it, and PublishState drops the lock before SetState. ShutdownAsync then posts its completion immediately, so an identify restart can apply the old context over the new cache or clobber the replacement source's status.

Additional Locations (2)

• libs/client-sdk/src/data_sources/fdv2/fdv2_data_source.cpp#L236-L252
• libs/client-sdk/src/data_sources/fdv2/fdv2_data_source.cpp#L191-L200

 

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[cursor]

Empty changeset marks source valid

Medium Severity

ApplyResult publishes kValid for every changeset, including a kNone result that carries no selector and no flags. That leaves initializing before a basis exists. The status manager then treats later errors as real interruptions, and a StartAsync waiter can resolve as successful before ReportExhausted moves the source to kShutdown.

Additional Locations (1)

• libs/client-sdk/src/data_sources/fdv2/fdv2_data_source.cpp#L245-L257

 

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