Realtime: ASR EOU language tags leak into the LLM conversation (unintended reply-language switching) #12748

Bug

ASR backends like Nemotron-3.5 append end-of-utterance language tags to
transcripts ("Hallo. <en-US>", "I would like to cancel my contract. <en-US> Can you help me with that? <de-DE>").

In the realtime pipeline the committed transcript is used verbatim for the
LLM conversation (the user item's Transcript field, which becomes the user
message in the LLM request). The per-utterance tag is unreliable for short
or ambiguous utterances (a German "Hallo." is tagged <en-US>; the same
uplink clip transcribes as "My name is Benjamin Zimmer. <en-US>" in one
run and "Mein Name ist Benjamin Zimmer. <de-DE>" in another). LLMs treat
the tag as a language signal and switch the reply language against the
session instructions — observed: a German call where the caller says
"Hallo" (tagged <en-US>) gets the reply "Hello. What is your phone number?".

Reproduction

1. Realtime session with input_audio_transcription on a Nemotron-3.5
   (or similar) backend.
2. Commit a short ambiguous utterance whose EOU tag does not match the
   conversation language.
3. The LLM reply switches language (or the reply behavior becomes
   tag-dependent), even though the instruction is to keep the conversation
   language.

Proposed fix

Strip the EOU language tags from the transcript that goes to
appendUserItem/generateResponse (the LLM context). The emitted
input_audio_transcription event keeps the raw transcript (debugging /
client display). PR follows.