fix: coerce null variations + FBEvent Flow + per-flag isolation on sync

[Devild007]

Summary

Three resilience fixes surfaced by real-world testing against a live FeatBit deployment.

1. coerceInputValues = true on both Json configs (StreamingJson + FbApiClient.json).
   Server occasionally ships "variation": null in streaming payloads for json-typed
   flags with an unset variation slot. Without coercion the whole payload fails to decode
   with Expected string value for a non-null key 'variation', got null literal,
   startTask never completes, and FBClient stays not-ready for its whole lifetime.
   With coercion, null coerces to the property default ("") and streaming proceeds.

2. Per-flag try/catch around store.upsert in both StreamingDataSynchronizer and
   PollingDataSynchronizer.safePoll. If a single flag entry slips past coercion (or throws
   for any other reason), we log-and-skip that entry instead of aborting the whole batch.
   FBClient still becomes ready with the surviving flags.

3. New FBClient.events(): Flow<FBEvent> API. Applications subscribe to surface
   transport / decode failures (Ready, Reconnecting, SyncError, TransportError) into
   their own observability stack (Sentry, Timber). SyncError.recoverable disambiguates
   one-bad-flag-in-batch from a full sync outage. README updated with the collector pattern.

Version bumped to 0.1.1-SNAPSHOT so JitPack consumers requesting this branch get a distinct
artifact.

Verification

• ./gradlew :featbit-client:build — green
• ./gradlew test — green
• Smoke-tested end-to-end against a live FeatBit dev environment (Fluent Health):
  before the fix, FBClient failed to start within 5s on every launch due to the null
  variation crash; after the fix, FBClient successfully started in ~0.6s and the
  downstream Android app receives the expected flag variation string.

Test plan

• Reviewer confirms coerceInputValues semantics: null on a defaulted non-null
  String field → property default rather than throw.
• Reviewer confirms FBEvent shape matches expected observability surface (Timber /
  Sentry mapping shown in README).
• JitPack produces v0.1.1 after merge + tag.

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