chore: schedule FDv2 recovery orchestration after fallback TTL expires

[joker23]

Summary

Wires the FDv2-recovery primitives from the previous PR in this stack into FDv2DataSource's synchronizer run loop: after falling back to FDv1, a recovery condition races alongside the existing fallback/recovery conditions, using the server-supplied TTL (or a caller-configurable override, or a 1-hour default if absent -- a TTL of 0 means indefinite, no recovery scheduled).

On firing, SourceManager.fdv2Recovery() re-enables FDv2 and the loop naturally attempts a fresh FDv2 connection -- if the server still signals fallback on that attempt, fallback re-engages from the new result. Also guards against the FDv1 synchronizer's own results re-triggering the fallback machinery. Ships with regression tests covering TTL present/absent/zero and the no-re-trigger guard.

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Note

Overview
Adds a guard in handleFdv1Fallback so when the active synchronizer is already the FDv1 fallback slot, results that still carry fdv1Fallback no longer call sourceManager.fdv1Fallback() again. That stops redundant fallback churn (e.g. spinning FDv2 back up or re-blocking slots) while the SDK is already on FDv1.

A regression test (SDK-2617) covers FDv2 signaling fallback, then the FDv1 synchronizer emitting both fallback-flagged and normal changeSets, and asserts FDv2 is only created once and both FDv1 payloads are delivered.

Also clarifies the synchronizer loop comment: condition timers must be closed on throw or early break, not only on normal paths.

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[joker23]

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[joker23]

Folding this into #1856. After removing the Condition-based FDv2 recovery primitive per review (#1856 (comment)), this layer's only remaining content was the FDv1-fallback re-trigger guard in handleFdv1Fallback -- which depends directly on #1856's isCurrentSynchronizerFDv1Fallback primitive. Folded that guard + its regression test into #1856 rather than keeping this as a near-empty separate PR. #1858 now rebases directly on top of #1856.