Remove unused generation wrapper traits

[gold-silver-copper]

Summary

• Remove unused public wrapper traits AudioGeneration<M>, ImageGeneration<M>, and Transcription<M>.
• Keep the live AudioGenerationModel, ImageGenerationModel, and TranscriptionModel APIs, request builders, clients, providers, examples, and tests intact.
• Document the public API removal as breaking in the root and rig-core changelogs.

Closes #2072.

Validation

• cargo fmt
• cargo fmt --check
• cargo check -p rig-core --all-features
• cargo check -p rig-core --features image,audio
• cargo test -p rig-core --features image,audio provider_response_tests
• cargo test -p rig-core --all-features provider_response_tests
• git diff --check
• exact rg / git grep checks confirmed the removed wrapper traits are gone and the live model traits plus Gemini Nano Banana support remain.

Not run

• Full workspace cargo test
• Full cargo clippy --all-targets --all-features
• cargo doc
• Live provider tests requiring credentials/network