fix(gateway): respect streaming edit intervals after buffer threshold

[zikaclawbot]

Summary

• Prevent the cumulative buffer threshold from bypassing the configured edit interval after the first streaming update.
• Preserve adaptive backoff, draft streaming, immediate final responses, and segment boundaries.
• Add six deterministic regressions exercising the actual consumer loop.

Verification

• The six regression cases failed against the base and passed after this change.
• Relevant stream-consumer, final-delivery and flood-retry test batches passed locally; independent review and CI are pending.
• No live gateway, configuration, or deployment was modified.

Scope

The existing upstream durable flood-retry path is reused, not duplicated. This change addresses excessive intermediate updates; it does not claim to eliminate Telegram-wide rate limits or every possible interruption.

Related: #54331. This narrower independent fix honors the configured/adaptive interval rather than a fixed one-second minimum. No code was copied from that PR.