feat: deprecate FDv2 payload filter builder methods

[beekld]

Deprecates the FDv2-specific payloadFilter builder methods, since payload filtering is not supported with the FDv2 data system.

Closes SDK-2971, part of SDK-2575.

• FDv2StreamingSynchronizerBuilder.payloadFilter, FDv2PollingSynchronizerBuilder.payloadFilter and FDv2PollingInitializerBuilder.payloadFilter are now @Deprecated.
• Each FDv2 builder now logs a warning at build time when a payload filter is configured, matching feat: deprecate FDv2 payload filter builder methods go-server-sdk#440.
• No behavior change otherwise: the filter is still applied for the deprecation period, and the FDv1 StreamingDataSourceBuilder/PollingDataSourceBuilder filter APIs are untouched.
• Removal (and dropping the dataSystem.payloadFilter wiring in the contract test service) is left for a follow-up.

Implementation details

The contract test harness removed the FDv2 payload filter scenarios (launchdarkly/sdk-test-harness#431), so the public FDv2 filter surface is going away across SDKs. This PR is the deprecation step only, keeping the change non-breaking; the contract test service still maps dataSystem.payloadFilter into these builders, which is why they aren't removed yet.

Unlike Go, Java never rejected an empty filter string (DefaultFDv2Requestor and StreamingSynchronizerImpl already skip null/empty), so no validation change was needed here.

New DataSystemComponentsTest covers the warning for all three builders and its absence when no filter is set. Verified with ./lib/sdk/server/gradlew -p lib/sdk/server test --tests '*DataSystemComponentsTest*' and compileJava javadoc.

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

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Note

Overview
Marks payloadFilter on the FDv2 polling initializer, polling synchronizer, and streaming synchronizer builders as @Deprecated, with Javadoc stating FDv2 does not support payload filtering and the API will be removed later.

When any of those builders build() with a non-null filter, DataSystemComponents now logs a WARN that filtering is unsupported and will stop being applied in a future release (aligned with the Go server SDK). Runtime behavior is unchanged for now—the filter is still passed through to requestors/synchronizers.

Adds DataSystemComponentsTest to assert the warning appears for all three builders when a filter is set, and does not appear when it is not. FDv1 data source payloadFilter APIs are not modified in this PR.

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