Reason: add target provider/model to existing provider registry

Add MiniMax provider and model support across the native and agent registries:

• Add MiniMax-M3 and MiniMax-M2.7 model defaults.
• Support global and China endpoint pairs for completions-compatible and messages-compatible configuration.
• Use chat completions for the completions-compatible path and append /v1/messages to the configured messages-compatible base URL.
• Update configuration examples and provider documentation.
• Add provider selection and request-path tests.

Checks:

• cargo fetch --locked
• cargo test -p codegraph-ai
• Compatibility-feature test suites for the native AI package
• cargo test -p codegraph-core
• Compatibility-feature test suites for the agent package
• rustfmt --edition 2021 on the modified Rust files
• git diff --check
• Configured secret scan