fix(gateway): reduce false duplicate-send alarms

[hermes-slape2]

Summary

• Classify stream consumers with no final-delivery signal as normal-final-send cases, not duplicate-send warnings.
• Retain warning-level diagnostics only when a delivery signal exists but cannot prove the completed reply.
• Add regression coverage for the no-signal path.

Why
All 34 alerts in the investigated window had streamed=false, previewed=false, and content_delivered=false. The normal final send was therefore the sole delivery path; a duplicate was impossible from that state.

Validation

• uv run ruff check gateway/run_turn.py tests/gateway/test_suppression_contract_matrix.py
• uv run pytest tests/gateway/test_suppression_contract_matrix.py -v (40 passed, 1 expected xfail)

[grantslape]

Please reach out to me directly regarding this PR. Sorry if this is causing noise.