Voice mode: all bundled ASR models fail silently — MultiModalProcessor routing bug for nemotron_speech (RNNT) in Foundry Local Core

### Describe the bug

`/voice` records audio successfully (level meter reacts, mic capture confirmed via raw PulseAudio capture) but **every transcription comes back empty**, for **all three** models offered in the `/voice` model picker:
- `nemotron-3.5-asr-streaming-0.6b`
- `nemotron-speech-streaming-en-0.6b`
- `nemotron-speech-streaming-es-0.6b`

All three share the same `nemotron_speech` (RNNT) architecture, so switching models does not help — there is no working model in the current picker.

### Root cause (traced to source)

The Foundry Local Core native audio transcription path throws on every call:

```
Microsoft.ML.OnnxRuntimeGenAI.OnnxRuntimeGenAIException: MultiModalProcessor cannot be created. nemotron_speech is not a registered multi-modal model type.
at Microsoft.ML.OnnxRuntimeGenAI.Result.VerifySuccess(IntPtr)
at Microsoft.ML.OnnxRuntimeGenAI.MultiModalProcessor..ctor(Model)
at Microsoft.AI.Foundry.Local.AudioClient.Transcribe(String, String, Nullable`1)
```

This is **not** a stale/outdated `onnxruntime-genai` engine issue. I downloaded and compared the public `onnxruntime-genai` v0.14.0 release (published 2026-05-29, after the "Multilingual Streaming Nemotron ASR" PR merged 2026-05-22) against the bundled runtime (labeled `0.14.1` in `deps_versions.json`, but not a publicly-tagged release). Both contain full, identical `NemotronSpeechModel`/`NemotronSpeechState` support (confirmed via symbol inspection of `libonnxruntime-genai.so`).

Cross-checking against `microsoft/onnxruntime-genai`'s `src/models/model.cpp` (https://github.com/microsoft/onnxruntime-genai/blob/main/src/models/model.cpp) confirms this is architecturally expected:

- `nemotron_speech` is an **RNNT-type** model (`ModelType::IsRNNT`) and is correctly routed to `NemotronSpeechModel` when the model is **loaded** (`CreateModel()` in `model.cpp`) — this is why `Model loaded successfully: nemotron-3.5-asr-streaming-0.6b-generic-cpu:3` appears in the Foundry log and the model shows as loaded.
- However, `MultiModalProcessor`'s constructor uses a **separate, hardcoded `processor_factory_` registry** that only contains vision+text chat model types (e.g. `phi4mm`, `gemma4`, etc.) — RNNT/TDT/ALM (Whisper) audio model types were never meant to go through `MultiModalProcessor` at all.

So the bug is that **`Microsoft.AI.Foundry.Local.Core`'s `AudioClient.Transcribe`/streaming session path unconditionally constructs a `MultiModalProcessor` for audio transcription**, instead of dispatching RNNT/TDT/ALM model types (`nemotron_speech`, Parakeet TDT, Whisper) through their own dedicated processing path (the same one used successfully at model-load time). Since `Microsoft.AI.Foundry.Local.Core.so` is a closed-source component, this can't be worked around or patched client-side — no `onnxruntime-genai` version swap fixes it, since the engine itself already works correctly and simply refuses (correctly, by its own design) to construct a `MultiModalProcessor` for a non-multimodal-chat model type.

### Affected version

GitHub Copilot CLI 1.0.69-0 (also reproduced on 1.0.66-1 — same bundled runtime pins: `onnxruntime-genai` 0.14.1, `foundry-local-core` 1.2.3, so the bug is not CLI-version-specific).

### Steps to reproduce the behavior

1. On Linux x64 (reproduced under WSL2/WSLg, but likely affects any Linux x64 install), run `/voice`, enable it, select any of the three offered models.
2. Press the voice-record shortcut, speak clearly for several seconds (confirmed via raw `parec` capture that real speech is captured at healthy signal levels, and the in-app level meter visibly reacts).
3. Stop recording.

### Expected behavior

Spoken audio is transcribed to text and inserted into the input box.

### Actual behavior

No error is shown to the user; the recording UI behaves normally, but the transcript is always empty. `~/.github-copilot-cli/logs/foundry.core*.log` shows:
```
[ERR] Error executing audio_transcribe: Microsoft.ML.OnnxRuntimeGenAI.OnnxRuntimeGenAIException: MultiModalProcessor cannot be created. nemotron_speech is not a registered multi-modal model type.
...
Audio stream stop: session , final flush text: '', full transcript: ''
```

### Additional context

- Confirmed mic capture and PulseAudio pipeline are fully healthy (not a WSLg/audio-hardware issue) via direct raw `parec -d RDPSource` capture showing strong, correctly-timed speech signal.
- Confirmed via a standalone script using the bundled `foundry-local-sdk` package directly (bypassing the Copilot CLI JS entirely) that `audioClient.transcribe()` throws the identical `MultiModalProcessor`/`nemotron_speech` error against a real captured WAV file, ruling out any Copilot-CLI-specific bug in the JS layer.
- Suggested fix: in Foundry Local Core's audio transcription path, dispatch by `ModelType::IsRNNT/IsTDT/IsALM` (mirroring `CreateModel()`'s dispatch in `onnxruntime-genai`'s `model.cpp`) instead of unconditionally constructing a `MultiModalProcessor`.

Start with the Foundry Local Core AudioClient.Transcribe and streaming session path described in the report, then compare its routing with microsoft/onnxruntime-genai/src/models/model.cpp and the CreateModel dispatch. Done means the affected RNNT, TDT, and ALM models use their dedicated processing paths and /voice returns transcription instead of an empty result.

Scritto dal modello di indicizzazione a partire dal testo della issue.