feat: add transcription and speech model support to provider registry

[lgrammel]

background

we want transcription and speech models to work in the registry like other model types do.

summary

• add transcriptionModel and speechModel to provider registry
• fix transcription providers to work with registry

verification

• all tests pass including new registry-specific tests for transcription/speech models
• autocomplete works for provider names and model IDs
• runtime errors for unsupported model combinations
• elevenlabs registry integration confirmed working

tasks

• make transcriptionModel and speechModel optional in ProviderV2
• add transcriptionModel and speechModel methods to registry
• update error handling for new model types
• convert transcription providers from Pick to full ProviderV2
• add proper error messages for unsupported model types
• test coverage for registry integration

future work

• consider adding more transcription/speech providers to registry examples