fix(gateway): transcribe voice messages during active agent runs

Salvaged from #6600 (@kristianvast) — re-scoped to the voice half only and
rebased onto current main. The cascading-interrupt hang half of the original
PR landed independently in dd0d1222a, so this carries ONLY Problem 1.

When a voice/audio message arrives while the agent is busy on the same
session, it hit the interrupt path with empty text because STT only ran after
the running-agent guard — the voice was effectively lost. Now we transcribe
audio BEFORE signaling the agent (and on the fresh-message path), echo the raw
transcript back to the user (🎙️), and _enrich_message_with_transcription
returns (text, transcripts) so callers can echo. A new
_dequeue_pending_with_transcription drives the post-agent drain the same way.

Reapplied onto _prepare_inbound_message_text (inbound enrichment was extracted
from the inline dispatch block since the original PR).

Co-authored-by: Kristian Vastveit <kristian@agrointel.no>

tests/gateway/test_stt_config.py

@@ -47,7 +47,7 @@ async def test_enrich_message_with_transcription_surfaces_path_when_stt_disabled
         "gateway.run._probe_audio_duration",
         new=AsyncMock(return_value="0:12"),
     ):
-        result = await runner._enrich_message_with_transcription(
+        result, transcripts = await runner._enrich_message_with_transcription(
             "caption",
             ["/tmp/voice.ogg"],
         )
@@ -56,6 +56,7 @@ async def test_enrich_message_with_transcription_surfaces_path_when_stt_disabled
     assert "voice message" in result.lower()
     assert "(duration: 0:12)" in result
     assert "caption" in result
+    assert transcripts == []
 
 
 @pytest.mark.asyncio
@@ -69,13 +70,14 @@ async def test_enrich_message_with_transcription_omits_duration_on_probe_failure
         "gateway.run._probe_audio_duration",
         new=AsyncMock(return_value=None),
     ):
-        result = await runner._enrich_message_with_transcription(
+        result, transcripts = await runner._enrich_message_with_transcription(
             "",
             ["/tmp/voice.ogg"],
         )
 
     assert "/tmp/voice.ogg" in result
     assert "duration" not in result.lower()
+    assert transcripts == []
 
 
 @pytest.mark.asyncio
@@ -89,7 +91,7 @@ async def test_enrich_message_with_transcription_avoids_bogus_no_provider_messag
         "tools.transcription_tools.transcribe_audio",
         return_value={"success": False, "error": "VOICE_TOOLS_OPENAI_KEY not set"},
     ):
-        result = await runner._enrich_message_with_transcription(
+        result, transcripts = await runner._enrich_message_with_transcription(
             "caption",
             ["/tmp/voice.ogg"],
         )
@@ -97,6 +99,7 @@ async def test_enrich_message_with_transcription_avoids_bogus_no_provider_messag
     assert "No STT provider is configured" not in result
     assert "trouble transcribing" in result
     assert "caption" in result
+    assert transcripts == []
 
 
 @pytest.mark.asyncio