fix(telegram): report truncated mid-stream preview as partial delivery

[cober-sky]

Problem

During streaming (finalize=False), edit_message truncates oversized previews to fit Telegram's 4096 UTF-16 limit (#48648) but returns success=True with no truncation marker — including from the saturated-preview dedup skips. The stream consumer then records the full accumulated text in _last_sent_text, has_delivered_text() matches the final response against that stale bookkeeping, and the gateway suppresses the normal final send:

Suppressing normal final send ... (streamed=True previewed=True content_delivered=False)

The truncated preview becomes the only delivery the user ever sees — long answers arrive permanently cut off.

Fix

Reuse the existing partial_overflow contract (already documented on SendResult.raw_response): return success=False with raw_response={"partial_overflow": True, "delivered_prefix": ..., "last_message_id": ...} from every mid-stream truncation site:

• pre-flight truncation,
• the saturated-preview dedup skip,
• the reactive message too long retry,
• message is not modified on an already-truncated preview.

The consumer's existing partial_overflow branch switches into fallback-final mode and delivers the missing tail on completion — no consumer changes needed.

This also composes well with the saturated-preview dedup: the consumer stops progressive edits after the first partial result, so saturated previews no longer burn flood budget at all.

Tests

Existing truncation/dedup/reactive tests updated to the partial-delivery contract; new test covers the not-modified path. tests/gateway/test_telegram_format.py (110), tests/gateway/test_stream_consumer.py + tests/gateway/test_telegram_overflow_partial.py (126) — all pass.

Found and battle-tested running the Telegram gateway against a local Ollama brain with streaming enabled.

[teknium1]

Thanks for tracing this through the adapter and stream-consumer contracts. The premise is confirmed on current main: plugins/platforms/telegram/adapter.py:4293, :4309, :4358, and :4365 report successful delivery after a truncated preview, while gateway/stream_consumer.py:1804 records the full accumulated input on successful edits. gateway/stream_consumer.py:1827-1853 already consumes the proposed partial_overflow metadata and enables the fallback-final path.

The PR's four producer-site changes align with that existing contract, and its updated adapter regressions cover the pre-flight, dedup, reactive-overflow, and not-modified paths. The current surrounding code retains the same target blocks, so salvage should be mechanical despite line movement since the PR base.

Automated hermes-sweeper review.