Add MiniMax provider support

[octo-patch]

Reason: Add MiniMax as a configurable provider with current model metadata and endpoints.

Changes:

• Adds MiniMax provider selection, credential loading, and regional endpoint defaults.
• Registers MiniMax-M3 and MiniMax-M2.7 model support with their context windows.
• Enables MiniMax-M3 attachment routing for supported image and video inputs.

Checks:

• git diff --check
• go test ./...