Add MiniMax provider mapping to the model gate

[octo-patch]

Reason: Add the missing MiniMax text provider mapping and regional compatibility endpoints to the CI-wired model-version gate.

Changes

• Add a machine-readable MiniMax text model registry with the supplied model parameters and required regions.
• Validate model details, default metadata, and regional endpoint consistency from the existing model-version gate.
• Link the provider registry from the model-version reference.

Checks

• git diff --check
• bash -n scripts/check-model-versions.sh scripts/check-surface-sync.sh
• python3 scripts/check-model-providers.py .claude/references/model-providers.json
• Injected missing-region and default-metadata drift fixtures; both were rejected with exit code 1.
• ./scripts/check-surface-sync.sh