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Each retry in the SetOutboxWatermark loop now gets its own 5s context.
Previously a shared context could already be expired when the second or
third attempt ran, making the retries pointless.
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Fixes #1691

- pollOutbox() reused a 10s context for SetOutboxWatermark after event
  processing could take much longer, causing "outbox watermark write
  failed: context deadline exceeded". The watermark never advanced so
  the same 200 events were reprocessed every poll cycle, spiking CPU.
  Now uses an independent 5s context with up to 3 retries (200ms apart).

- When multiple Codex accounts sharing the same 21-22 groups are all
  rate-limited in quick succession, each account_changed event triggered
  redundant bucket rebuild attempts for the same groups. Introduce
  batchSeenKey{groupID, platform} and thread a seen map through the
  handler chain; rebuildBucketsForPlatform skips (group, platform) pairs
  already rebuilt within the same poll batch (~80% fewer rebuild calls in
  the 5-accounts-same-groups scenario).
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

refactor: extract ReadUpstreamResponseBody to deduplicate upstream response read + too-large error handling

Consolidates 9 call sites of resolveUpstreamResponseReadLimit + readUpstreamResponseBodyLimited + ErrUpstreamResponseBodyTooLarge error handling into a single ReadUpstreamResponseBody function with TooLargeWriter callback for API-format-specific error responses (Anthropic, OpenAI, countTokens).