Three fixes for the duplicate reply bug affecting all gateway platforms:
1. base.py: Suppress stale response when the session was interrupted by a
new message that hasn't been consumed yet. Checks both interrupt_event
and _pending_messages to avoid false positives. (#8221, #2483)
2. run.py (return path): Remove response_previewed guard from already_sent
check. Stream consumer's already_sent alone is authoritative — if
content was delivered via streaming, the duplicate send must be
suppressed regardless of the agent's response_previewed flag. (#8375)
3. run.py (queued-message path): Same fix — already_sent without
response_previewed now correctly marks the first response as already
streamed, preventing re-send before processing the queued message.
The response_previewed field is still produced by the agent (run_agent.py)
but is no longer required as a gate for duplicate suppression. The stream
consumer's already_sent flag is the delivery-level truth about what the
user actually saw.
Concepts from PR #8380 (konsisumer). Closes#8375, #8221, #2483.
