feat(core): integrate RetryManager into SegmentDestination upload pipeline

[abueide]

Summary

• Wire RetryManager into SegmentDestination for TAPI-compliant retry handling
• Add uploadBatch() with error classification, retry-after parsing, and configurable retry behavior
• Add aggregateErrors() to separate batch results by status (success/429/transient/permanent)
• Add updateRetryState() that delegates to RetryManager and returns whether limits were exceeded
• Implement per-event age pruning via _queuedAt timestamps and pruneExpiredEvents() — events older than maxTotalBackoffDuration are dropped individually
• Add canRetry() upload gate check before sending batches
• Add X-Retry-Count header propagation to API requests
• Replace QueueFlushingPlugin.dequeue() with dequeueByMessageIds() for messageId-based dequeue
• Replace isPendingUpload flag with promise-based concurrent flush guard
• Override shutdown() (instead of standalone destroy()) to integrate with plugin lifecycle — prevents auto-flush timer leak on client cleanup
• Handle RetryResult.limit_exceeded: log warning and let per-event age pruning handle drops (events are NOT bulk-dropped on global counter reset)
• Add comprehensive tests for pruning logic, retry integration, and API retry count

PR 5 of 5 in the TAPI backoff/retry stack. Depends on #1159.

Test plan

• All 42 SegmentDestination tests pass
• All 461 total tests pass
• shutdown() properly cleans up RetryManager timers via plugin lifecycle
• Events only dropped by per-event _queuedAt age, not by global retry counter reset

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