feat(minimax): add M3 model constant

[octo-patch]

Reason: Add the missing MiniMax-M3 model constant to Rig's existing MiniMax provider registry.

Summary:

• Add the public MINIMAX_M3 model identifier alongside the existing MiniMax constants.
• Add a unit test that locks the exported identifier to the provider model ID.

Checks:

• cargo fmt --all -- --check
• cargo test -p rig-core providers::minimax