feat(studio): add MiniMax as an external LLM provider

[octo-patch]

Summary

Add MiniMax as a new external LLM provider in Unsloth Studio, alongside the existing OpenAI-compatible providers (DeepSeek, Kimi, Qwen, …).

Changes

• Add a minimax entry to PROVIDER_REGISTRY in studio/backend/core/inference/providers.py. It uses MiniMax's OpenAI-compatible endpoint (https://api.minimax.io/v1) with Bearer auth, so it routes through the existing /chat/completions path like the other OpenAI-compatible providers — no new backend translation needed.
• Seed the model picker with MiniMax-M3 (1M-token context, image input).
• Update tests/test_providers_api.py: register MiniMax in the provider config map and bump the expected registry size (9 → 10).

Testing

• python -m py_compile on both changed files.
• Imported the registry and asserted the new entry resolves (get_provider_info/get_base_url), routes through the OpenAI-compatible path, and that list_available_providers() returns 10 entries all carrying the required fields.
• ruff check passes on the changed files.
• The test_providers_api.py suite is a live-server integration test (skipped without a running Studio instance / API key), so the registry wiring is covered by CI.

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[gemini-code-assist]

Code Review

This pull request adds support for the MiniMax inference provider, updating both the provider registry and the test suite. The review feedback suggests adding the China mainland base URL to the provider notes for consistency and including MiniMax in the vision test parameters to ensure its vision capabilities are properly covered.

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[gemini-code-assist]

For consistency with other China-based providers (like Kimi and Qwen), it is highly beneficial to mention the China mainland base URL (https://api.minimaxi.com/v1) in the notes field.

Suggested change

	"notes": "OpenAI-compatible API. API key from platform.minimax.io. MiniMax-M3 has a 1M-token context window and image input.",
	"notes": "OpenAI-compatible API. API key from platform.minimax.io. MiniMax-M3 has a 1M-token context window and image input. China mainland: override base URL to https://api.minimaxi.com/v1",

[gemini-code-assist]

Since the minimax provider supports vision (supports_vision: True in the registry), it should also be included in the _VISION_PARAMS list (around line 480) so that its vision capabilities are covered by the integration tests.