Add MiniMax email compose provider

[octo-patch]

Reason: The registered email compose and subject-generation endpoints need a configurable MiniMax implementation instead of inert stubs.

This change:

• adds MiniMax-backed compose and subject generation
• supports MiniMax-M3 and MiniMax-M2.7 through the global and CN OpenAI-compatible endpoints
• validates and forwards the provider configuration for managed email deployments
• adds provider request and route coverage

Checks:

• git diff --check origin/main..HEAD
• secret scan against the committed upstream diff