feat: expand MiniMax provider registry

[octo-patch]

Reason: add target provider/model to existing provider registry.

• Register MiniMax-M3 and MiniMax-M2.7 with the configured model metadata.
• Expose global_en and cn_zh OpenAI-compatible and Anthropic-compatible endpoint choices in the provider manifest and LLM settings form.
• Route configured MiniMax Anthropic-compatible bases through the native Anthropic client while preserving the OpenAI-compatible path.
• Add focused registry and client-routing regression tests.

Checks:

• git diff --check
• ruff check backend/app/services/llm/client.py backend/tests/test_llm_provider_registry.py
• PYTHONPATH=. pytest tests/test_llm_provider_registry.py
• npm run build --prefix frontend

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Normalize MiniMax Anthropic URLs before client selection

When an admin enters the full MiniMax Anthropic endpoint ending in /v1/messages (or /v1), this exact comparison returns false even though AnthropicClient._normalize_base_url() already supports those forms. The model then falls through to OpenAICompatibleClient, which appends /chat/completions and sends the wrong protocol to a URL like .../anthropic/v1/messages/chat/completions; normalize the candidate URL the same way before matching, or accept the documented full endpoint variants.

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