[abelonogov-ld]

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

Related issues

Mirrors payloadFilterKey in the React Native and browser SDKs (js-core#551) and follows the filtered environments SDK spec. iOS counterpart: ios-client-sdk#548.

Describe the solution you've provided

Adds LDConfig.Builder.payloadFilter(String). When set, the SDK adds filter=<key> to every flag data request:

• FDv1 streaming (/meval, GET and REPORT)
• FDv1 polling (/msdk/evalx, GET and REPORT), including the FDv1 fallback synchronizer in the data system
• FDv2 streaming and polling (the DefaultFDv2Requestor already accepted a filter but was always passed null)

The key is checked when the config is built. A null or empty key means no filter. A key that doesn't match ^[a-zA-Z0-9][._\-a-zA-Z0-9]*$ is dropped with an error log, and the full environment is requested, as the spec requires (filtering-strict behavior).

The value reaches FDv1 data sources through ClientContext.getConfig() and FDv2 builders through the internal DataSourceBuildInputsInternal, so there are no changes to the public ClientContext or DataSourceBuildInputs APIs.

The contract-test service now maps the harness's streaming.filter / polling.filter to the new option and declares the filtering and filtering-strict capabilities.

Describe alternatives you've considered

• Putting the option on StreamingDataSourceBuilder / PollingDataSourceBuilder, as the Java server SDK does. A single top-level option matches React Native and covers the FDv2 data system without changes to each builder.
• Sending invalid keys as-is (React Native's current behavior). Rejected in favor of the spec.

Additional context

This PR only sends the parameter; the SDK does no filtering of its own and doesn't change how it picks streaming or polling. The server doesn't currently apply the filter on every mobile endpoint:

• FDv1 polling (/msdk/evalx): the streamer drops the query string before it reaches the origin, so these responses are unfiltered.
• FDv1 streaming (/meval): the filter is forwarded and applied to the initial put when the payload-filter-gonfalon-sdk-endpoints flag is on. Live updates are only routed by filter when payload-filter-streamer-updates is on.

So an app that switches between streaming and polling may see the full flag set after a poll. The React Native SDK has the same limitation today.

I haven't run the contract test harness against this branch locally. All 764 unit tests pass.