feat: add FDv2 configuration builder

[beekld]

Adds a public FDv2Builder and wires the FDv2 data system into MakeDataSystem, so end users can opt into the FDv2 protocol via the existing config-builder pattern.

// Default FDv2 stack (streaming primary, polling fallback,
// FDv1 streaming last-resort)
auto config = ConfigBuilder("sdk-key-123")
    .DataSystem().Method(DataSystemBuilder::FDv2())
    /* ... */;

// Customised: payload filter + shorter fallback timeout, no FDv1 fallback
auto config = ConfigBuilder("sdk-key-123")
    .DataSystem().Method(
        DataSystemBuilder::FDv2()
            .Streaming(FDv2Builder::StreamingSource().Filter("mobile-flags"))
            .FallbackTimeout(std::chrono::seconds{30})
            .DisableFDv1Fallback())
    /* ... */;

Test plan

• 5 new FDv2 builder tests in config_builder_test.cpp
• Full server suite green (503 tests)

---

Note

Medium Risk
Changes how the client selects and instantiates flag data sources (new FDv2 path and factory wiring); misconfiguration could affect connectivity and fallback behavior, though default remains background sync.

Overview
FDv2 is now a selectable data system via DataSystemBuilder::Method(FDv2Builder), alongside background sync and lazy load. Public FDv2Builder (Default() / Custom()) configures polling initializers, ordered streaming/polling synchronizers, optional FDv1 fallback, and fallback/recovery timeouts; built config adds FDv2Config with per-source filter and BaseUrl overrides.

Runtime wiring: MakeDataSystem constructs FDv2DataSystem through new initializer and synchronizer factories (FDv2 polling/streaming plus FDv1 adapter factories). FDv2 poll/stream components now take a resolved base URL string (override or service endpoints) instead of full ServiceEndpoints.

Tests cover FDv2 builder defaults, synchronizer ordering, FDv1 fallback, timeouts, and base URL overrides; existing FDv2 HTTP tests updated for the new poll-request API.

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[kinyoklion]

The FDv2 endpoints don't support filters as far as I am aware. Instead we are depending on view keys.

[beekld]

Maybe I'm misunderstanding. Isn't that what these are?

https://github.com/launchdarkly/java-core/blob/ad2ac08c571ef067f3df79a2c0774229c722cc90/lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/StreamingSynchronizerImpl.java#L124-L127

https://github.com/launchdarkly/java-core/blob/ad2ac08c571ef067f3df79a2c0774229c722cc90/lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/integrations/FDv2StreamingSynchronizerBuilder.java#L81

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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