chore: Pipe headers through data sources.

[kinyoklion]

This pipes the fdv1 fallback header, and the environment ID, through the data sources. It doesn't utilize either piece of information.

This is draft while I sort publishing the okhttp-evensource.

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Note

Introduces header propagation and response handling improvements across FDv2 data sources.

• Adds HeaderConstants and plumbs x-ld-fd-fallback and x-ld-envid from HTTP/stream events into FDv2SourceResult (new fdv1Fallback flag) and ChangeSet environment ID
• Refactors FDv2Requestor.FDv2PayloadResponse to include isSuccess, statusCode, and factory methods (success, failure, none); 304 now returns a non-null "none" response
• Updates DefaultFDv2Requestor, PollingBase, PollingInitializerImpl, and StreamingSynchronizerImpl to use structured responses and extract headers for error handling and changeset conversion
• Extends unit tests to cover header propagation, new response semantics, ETag/304 handling, and error paths; replaces exception-based HTTP error assertions with response-based checks
• Bumps dependency okhttp-eventsource to 4.2.0

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[kinyoklion]

Instead of having null indicate no transfer I made an explicit none.

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