feat: Add FDv2 data system support to NodeClient

[joker23]

SDK-2468

---

Note

Medium Risk
Touches flag data ingestion and connection/offline transitions for FDv2-enabled clients; FDv1 is isolated but incorrect mode or flush handling could affect live updates and events.

Overview
NodeClient can use the shared FDv2 data stack when dataSystem is configured, while the existing FDv1 NodeDataManager path stays unchanged.

Construction wires createFDv2DataManagerBase with desktop defaults, transition table, and a buildQueryParams hook (auth / h for client-side ID; empty params and hash warnings for mobile key). Connection mode is tracked on the client (getConnectionMode / isOffline); FDv2 setConnectionMode validates modes, no-ops after close or on duplicate mode, serializes concurrent transitions (flush before offline, then event sending toggles), and still delegates to the FDv2 data manager.

CI runs FDv1 contract tests without stopping the harness service, then adds a FDv2 harness run (v3) with testharness-suppressions-fdv2.txt. The contract test service advertises client-event-source-http-errors and documents deferred fdv1-fallback support. Unit coverage lives in NodeClientFDv2.test.ts.

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[github-actions]

@launchdarkly/js-sdk-common size report
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@launchdarkly/js-client-sdk size report
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Uncompressed size: 114248 bytes

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[joker23]

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[joker23]

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