Add sound generation support

[saarnilauri]

Summary

• Introduce SoundGenerationModelInterface and SoundGenerationOperationModelInterface contracts for sound generation models
• Add SOUND_GENERATION capability to CapabilityEnum
• Wire sound generation into PromptBuilder and AiClient with full test coverage

Why this is needed?

Why a separate SOUND_GENERATION capability instead of reusing SPEECH_GENERATION?

Speech and sound are fundamentally different audio domains with different provider landscapes, parameters, and use cases:

1. Different output types — Speech models produce spoken language (voice, narration, dialogue). Sound models produce non-verbal audio (sound effects, ambient noise, foley). A consumer asking for "rain on a tin roof" or "sword
   clash" should not be routed to a speech model.
2. Different provider models — AI providers expose these as distinct model types (e.g. ElevenLabs sound effects vs. their TTS voices, or dedicated SFX models). Collapsing them into one capability would force providers to
   either advertise a capability they don't fully support or implement awkward routing logic internally.
3. Consistent with the existing pattern — The client already distinguishes between TEXT_TO_SPEECH_CONVERSION (deterministic TTS), SPEECH_GENERATION (generative voice/dialogue), and MUSIC_GENERATION. Sound generation is a
   natural sibling — it completes the audio capability taxonomy without overlap. Merging sound into speech would break the 1:1 mapping between capability enum values and provider model interfaces that the rest of the
   architecture relies on.
4. Clean capability discovery — Consumers can query CapabilityEnum::soundGeneration() to find providers that actually support sound effect generation, rather than getting speech models that can't fulfill the request.

Test plan

• Unit tests added for PromptBuilder sound generation methods
• AiClient test updated to cover the new capability
• CapabilityEnum test updated for SOUND_GENERATION

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.17%. Comparing base (6317042) to head (ec0445b).

Files with missing lines	Patch %	Lines
src/Builders/PromptBuilder.php	95.23%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #222      +/-   ##
============================================
+ Coverage     88.12%   88.17%   +0.04%     
- Complexity     1213     1222       +9     
============================================
  Files            60       60              
  Lines          3934     3958      +24     
============================================
+ Hits           3467     3490      +23     
- Misses          467      468       +1     

Flag	Coverage Δ
unit	88.17% <95.83%> (+0.04%)	⬆️

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