fix(gateway): suppress silent placeholder replies in live chats

[luoxiao6645]

Summary

• suppress placeholder silent replies in the live gateway response path
• prevent (No message), (No reply), [SILENT], and similar exact silence markers from being sent to Discord and other live messaging platforms
• add focused regression coverage for silent-response normalization

Problem

In live gateway chats, the model can decide it should stay silent but still emit placeholder text such as (No message), (No reply), or [SILENT].

The gateway was treating those placeholders as normal text and delivering them to Discord instead of suppressing delivery entirely.

Root Cause

The live messaging response path had no normalization step for silence markers before adapter delivery. As a result, placeholder "no reply" outputs were forwarded like ordinary assistant messages.

Changes

• add _normalize_live_gateway_response() in gateway/run.py
• suppress exact silence markers before normal live response delivery
• suppress the same markers in the queued follow-up first-response path
• preserve real errors and non-placeholder content
• add regression tests covering placeholder and non-placeholder cases

Testing

• venv\\Scripts\\python -m pytest tests/gateway/test_live_silent_responses.py -q -n 0

Closes #9840

[luoxiao6645]

Closing this PR in favor of #9956.\n\nReason: #9956 is a clean, focused follow-up for #9840 that keeps the silent-placeholder fix and the streaming-path completion in one dedicated PR, without continuing review on the earlier thread.