feat(model): default MiniMax provider to MiniMax-M3

[octo-patch]

Reason: The MiniMax provider defaults stop at MiniMax-M2.7; bump both minimax and minimax-cn defaults to the current MiniMax-M3 model.

Changes

• src/model-spec.ts: updated the DEFAULT_MODEL_PER_PROVIDER entries for minimax and minimax-cn from MiniMax-M2.7 to MiniMax-M3.

These defaults are the model id buildFallbackModel selects when a requested model id is not found in the registry for a MiniMax provider, so they are the canonical place the current model is declared.

Checks

• Verified no tests, docs, generated registries, or guidance snapshots hard-code the previous MiniMax default (a repo-wide search for MiniMax only matches the two updated lines).
• No secret material in the diff.