feat: warn that payload filtering has no effect with FDv2

[beekld]

Documents that the shared payloadFilterKey option has no effect with the FDv2 data system, and logs a warning when it's configured alongside FDv2.

Closes SDK-2974, part of SDK-2575.

• payloadFilterKey remains fully supported for FDv1, so the option itself is neither deprecated nor removed.
• Server-side warning is emitted from constructFDv2; client-side warning from the shared createFDv2DataManagerBase (covers browser, react-native and node-client).
• The ?filter= query param is still sent on FDv2 requests during this deprecation period; the FDv2 backend path doesn't use it to select the payload.

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Implementation details

Related issues

Follows the same approach as the Go (launchdarkly/go-server-sdk#440) and Java (launchdarkly/java-core#209) deprecations, and the parallel Python (launchdarkly/python-server-sdk#518) / Ruby (launchdarkly/ruby-server-sdk#434) PRs. Payload-filter scenarios for FDv2 were removed from the test harness in launchdarkly/sdk-test-harness#431.

Describe the solution you've provided

• packages/shared/sdk-server/src/api/options/LDOptions.ts and packages/shared/sdk-client/src/api/LDOptions.ts: doc comments note that payload filtering isn't supported with FDv2.
• packages/shared/sdk-server/src/LDClientImpl.ts: warns in constructFDv2 when payloadFilterKey is configured.
• packages/shared/sdk-client/src/datasource/FDv2DataManagerBase.ts: warns when serviceEndpoints.payloadFilterKey is configured. Centralizing it here covers every client-side SDK, since browser, react-native and node-client all build on this manager.

Describe alternatives you've considered

• Deprecating or removing the option outright: rejected, since it's shared with FDv1 where it's still supported.
• Dropping ?filter= from FDv2 request URLs now: deferred to the removal phase so this change is behavior-preserving.

Additional context

Verified: yarn workspace @launchdarkly/js-server-sdk-common test (53 suites, 1022 passed) and lint; yarn workspace @launchdarkly/js-client-sdk-common test (57 suites, 866 passed) and lint.

Link to Devin session: https://app.devin.ai/sessions/ed604c6db94d452ea9f2dc35d6acc398
Open in Devin Desktop: https://app.devin.ai/desktop/session/ed604c6db94d452ea9f2dc35d6acc398?variant=devin
Requested by: @beekld

---

Note

Overview
When payloadFilterKey is set while using the FDv2 data system, the SDK now logs a warning that payload filtering is unsupported and has no effect on FDv2 requests. payloadFilterKey is unchanged for FDv1 and is not removed or deprecated.

Warnings are emitted at server FDv2 setup in constructFDv2 (LDClientImpl) and at shared client FDv2 setup in createFDv2DataManagerBase (browser, React Native, node-client). LDOptions doc comments on client and server now state the same limitation.

Tests cover both code paths (warn when a filter key is present, no matching warn when absent).

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