feat(minimax): add image generation support

[octo-patch]

Reason: MiniMax image generation was unavailable even though the provider exposes dedicated image generation models and endpoints.

Changes:

• enable image generation for the MiniMax client and export the image model constants
• map supported request fields to the dedicated image generation endpoint for global and China clients
• handle base64 and URL responses without forwarding authorization to image download URLs
• preserve HTTP failures and provider status errors with their original response bodies

Checks:

• cargo fmt --all -- --check
• cargo test -p rig-core --features image providers::minimax
• cargo clippy -p rig-core --features image --all-targets -- -D warnings