realtime: honor output_modalities to skip TTS in text-only mode (#9838)

* realtime: honor output_modalities to skip TTS in text-only mode

The emulated realtime pipeline previously ignored the OpenAI Realtime spec
field output_modalities and always synthesized TTS. Add resolveOutputModalities
+ modalitiesContainAudio helpers and gate the TTS / ResponseOutputAudio*
emission so a client requesting ["text"] gets only ResponseOutputText* events.

This lets thin clients (e.g. thing5-poc) cache TTS on the client side while
still using the realtime WS for VAD + STT + LLM + tool-call parsing.

Assisted-by: Claude:claude-opus-4-7

* realtime: plumb response-level output_modalities and echo on session

Follow-up to the previous commit:
- Resolve response.create's output_modalities at the gate so a per-response
  override of an audio session is honored (the test asserted this contract
  but the production call site was passing nil).
- Mirror OutputModalities in the RealtimeSession echo so session.update
  round-trips the client-supplied value, matching MaxOutputTokens's pattern.

Assisted-by: Claude:claude-opus-4-7

* realtime: silence errcheck on deferred os.Remove of TTS file

CI's errcheck flagged the pre-existing `defer os.Remove(audioFilePath)`
inside the audio-emission block (now wrapped by the modality gate). Wrap
the call in a closure that explicitly discards the error — the canonical
Go pattern for "I want to defer a cleanup whose error I genuinely don't
care about."

Assisted-by: Claude:claude-opus-4-7 golangci-lint

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Co-authored-by: Ettore Di Giacinto <mudler@localai.io>