Add Whisper provider for audio transcription

[JaimeStill]

Migrated from tailored-agentic-units/tau-agent#1

Summary

Add a WhisperProvider implementation for speech-to-text transcription through the agent subsystem's provider abstraction. This issue also addresses provider-level architectural changes discovered during the audio protocol implementation.

Scope

Provider Architecture Changes

• Header ownership: Move Content-Type ownership from the Request interface to the Provider. Remove Headers() from request types. Provider sets all protocol headers in PrepareRequest().
• Multipart encoding: AudioData.Input changes from string to []byte. BaseProvider.marshalAudio() produces multipart/form-data with boundary. Provider stores content-type for PrepareRequest().
• Protocol capability validation: Providers should declare supported protocols upfront. Clean error at config/creation time if a protocol is unsupported.

Whisper Provider Implementation

Important: onerahmet/openai-whisper-asr-webservice is NOT OpenAI-compatible. It requires a dedicated provider:

Aspect	OpenAI/Azure Standard	Whisper Container
Endpoint	/v1/audio/transcriptions	/asr
Field name	file	audio_file
Options	Request body fields	Query parameters
Model field	Required	Not used
Authentication	Bearer token / API key	None

• Create agent/providers/whisper.go embedding BaseProvider
• Implement Provider interface with dedicated endpoint, field mapping, and query parameter handling
• Register via Register("whisper", NewWhisper) in agent/providers/registry.go

Azure Whisper

• Azure uses standard OpenAI multipart format at /deployments/whisper/audio/transcriptions
• Azure's existing provider should work once multipart encoding is implemented in BaseProvider

Docker

• Add whisper service to docker-compose.yml using onerahmet/openai-whisper-asr-webservice:latest on port 9999:9000 with GPU support

CLI

• Extend cmd/prompt-agent/ with -input flag and executeAudio function

Configuration

• Add whisper configuration for local whisper container

Testing

• Black-box tests co-located with provider implementation
• Constructor, endpoint routing, request preparation, and response processing tests
• HTTP mocking with httptest.NewServer()

Notes

• Multipart form data handling is non-standard compared to existing JSON-based providers
• Follow existing Ollama/Azure provider patterns for interface implementation