fix(gateway): keep transcript echoes opt-in

[chriswinig]

Summary

• stops automatic raw STT transcript echo unless stt_echo_transcripts is explicitly enabled
• keeps transcript content prepended to the agent input, avoiding duplicate Telegram replies for transcribe/translate voice messages
• applies the same behavior to mid-turn voice interrupts

Test plan

• python -m compileall -q gateway

[teknium1]

Superseded by PR #58859, which makes the transcript echo configurable via stt.echo_transcripts instead of flipping it to opt-in unconditionally — hard-disabling the echo by default would silently change behavior for users who rely on it. The config route gives your deployment the same outcome (stt.echo_transcripts: false). Thanks for flagging the noise issue!