fix: make FDv1 fallback TTL-based and recover to FDv2 after the TTL

[beekld]

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 (unit tests only; not run on device/emulator)

Related issues

SDK-2991 (part of SDK-2935); spec change sdk-specs#251 (DATASYSTEM v2 1.6 / CSFDV2 8.3). Reference: cpp-sdks#631.

Describe the solution you've provided

FDv1 fallback is now TTL-based and never terminal.

• TTL is read from the X-LD-FD-Fallback-TTL header (polling and streaming) and from goodbye.protocolFallbackTTL; a goodbye carrying that field is itself a fallback directive. A fallback directive is now carried as a @Nullable FDv1FallbackTTL on FDv2SourceResult (getFdv1Fallback(); null means no directive). The existing boolean factories are kept and map true to the default TTL.
• A TTL in (0, 3600]s is used as-is; anything else (missing, malformed, 0, negative, > 3600) uses the 1 h default minus uniform [0, 1800]s jitter. This is resolved once, when FDv1FallbackTTL is parsed from the header or goodbye.
• On fallback, FDv2DataSource stops the FDv2 synchronizers (retained in SourceManager), runs the FDv1 synchronizer if configured, and schedules recovery after the TTL. Recovery blocks FDv1 again and rebuilds a fresh FDv2 synchronizer. A repeated directive while on FDv1 only reschedules the TTL.
• Fallback now happens even with no FDv1 synchronizer configured: no synchronizer runs (status INTERRUPTED) until the TTL elapses, then FDv2 restarts. Previously the directive was ignored in that case.
• Single writer: only one synchronizer is active at a time; the outgoing one is closed before the next is built, and SourceManager now closes the active source when nothing can replace it.

Implementation details

• SourceManager tracks which synchronizers fdv1Fallback() blocked, so recoverFromFdv1Fallback() unblocks only those (FDv2 sources that failed terminally stay blocked).
• A streaming goodbye that carries a fallback directive does not reconnect the stream, since the data source closes that synchronizer anyway; a plain goodbye still reconnects as before.
• The info log on fallback changed to include the TTL ("Server directed fallback to FDv1; FDv2 will be retried in N seconds.").
• Tests: FDv1FallbackTTLTest (valid TTLs unchanged, invalid TTLs land in [1800, 3600], header/goodbye parsing), header and goodbye propagation in DefaultFDv2RequestorTest, FDv2PollingBaseTest, FDv2StreamingSynchronizerTest, and FDv2DataSourceTest recovery with and without an FDv1 synchronizer (using a 1 s server TTL). ./gradlew :launchdarkly-android-client-sdk:testDebugUnitTest javadoc passes locally.

Describe alternatives you've considered

Treating "no FDv1 synchronizer" as "ignore the directive" (old behavior); rejected because the spec requires stopping FDv2 and recovering after the TTL regardless.

Additional context

Deliberately unchanged: the ordinary fallback/recovery condition timers (2 min / 5 min) between FDv2 synchronizers, and the contract tests (the harness has no coverage of fallback TTL).

Link to Devin session: https://app.devin.ai/sessions/b4e7a7ad5f06499695e4ce57a2ca6cc2
Open in Devin Desktop: https://app.devin.ai/desktop/session/b4e7a7ad5f06499695e4ce57a2ca6cc2?variant=devin
Requested by: @beekld

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