When the stream consumer's got_done handler successfully delivers the
final response content via _send_or_edit but the subsequent edit
(e.g. cursor removal) fails, final_response_sent remains False even
though the user has already received the final answer. The gateway's
fallback send path then re-delivers the same content, causing the
user to see the response twice on Telegram.
Introduce a new _final_content_delivered flag on the stream consumer,
set by the got_done handler when the final content has reached the
user. The _run_agent suppression logic now treats this flag as an
additional signal (alongside final_response_sent and
response_previewed) that final delivery is already complete.
This preserves the existing behavior for intermediate-text-only
streams (where already_sent=True but no final content has been
delivered) — those still receive the gateway's fallback send, matching
the test expectation in test_partial_stream_output_does_not_set_already_sent.
Adds TestFinalContentDeliveredSuppression with two cases covering
both the suppression (content delivered + edit failed) and the
non-suppression (intermediate text only) branches.
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