chore: adds FDv1 Fallback support to FDv2 Data Source

[tanderson-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
  Will be done as final integration testing

---

Note

Adds FDv1 fallback to FDv2 data sources, triggered by the x-ld-fd-fallback header and configured via SDK DataSystem to use a polling fallback synchronizer.

• FDv2 data source fallback:
  • Add FDv1FallbackActionApplier in FDv2DataSource to switch from FDv2 synchronizers to FDv1 synchronizers when ErrorInfo.FDv1Fallback is true.
  • Include optional FDv1 synchronizers composite in the combined CompositeSource.
• Error signaling:
  • Add DataSourceStatus.ErrorInfo.FDv1Fallback flag.
  • Set this flag in FDv2PollingDataSource and FDv2StreamingDataSource when the x-ld-fd-fallback header is present on HTTP errors.
• SDK config wiring (contract tests):
  • In SdkClientEntity, configure FDv1 polling fallback synchronizer derived from the selected synchronizer and align global polling ServiceEndpoints when a custom polling base URI is provided.
• Tests:
  • Add tests covering header parsing, fallback triggering, and FDv1 synchronizer activation across polling/streaming and composite behavior.
  • Update test-supressions-fdv2.txt to reflect new FDv1 fallback handling.
• Dependencies:
  • Bump LaunchDarkly.EventSource to 5.3.0 and LaunchDarkly.InternalSdk to 3.6.0.

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[tanderson-ld]

Need to update references pointing to EventSource of Internal before this merges.

[kinyoklion]

The couple comments that cursor bot made seem accurate, but if those are addressed, then it looks good.

[kinyoklion]

Can we pass any more contract tests now?