providers: add MiniMax preset

[octo-patch]

Reason: add MiniMax-M3 to provider/model registry.

Summary

• Add MiniMax to the provider type registry with MiniMax's Anthropic-compatible base URL and default API key env.
• Route MiniMax through Murph's OpenAI-compatible provider path using MiniMax's OpenAI base URL.
• Cover MiniMax provider loading, metadata secrets, and Murph env generation in config tests.

Test plan

• Secret scan on pending diff passed.
• bun test src/config/__tests__/providers.test.ts src/config/__tests__/loader.test.ts src/config/__tests__/loader-metadata.test.ts (not run: bun is not installed in this environment)

🤖 Generated with Claude Code

[octo-patch]

Updated the MiniMax preset so the Agent SDK base URL produces a single /v1/messages path, and added coverage for MiniMax-M3, MiniMax-M2.7, regional endpoints, and model details. I ran bun test (2,644 passed), bun run lint, and bun run typecheck.

[octo-patch]

This PR is replaced by #166, which keeps the same focused MiniMax provider change on a clean commit history. Continuing review at #166.