fix(provider): preserve MiniMax cache by default

[wgu9]

Issue for this PR

Closes #31755

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Stops forcing thinking: { type: "adaptive" } onto MiniMax M3 requests that use the Anthropic SDK path by default.

MiniMax users reported that cache reads work when the variant is explicitly none, but the current default path still injects adaptive thinking before any variant is selected. This makes default behave like thinking, which can burn through MiniMax token-plan quota and disable effective caching.

The explicit MiniMax M3 variants remain unchanged:

• none still sends thinking: { type: "disabled" }
• thinking still sends thinking: { type: "adaptive" }
• default now sends no thinking override

How did you verify your code works?

• bun test test/provider/transform.test.ts --test-name-pattern "minimax m3 thinking|minimax m3 using" from packages/opencode
• bun typecheck from packages/opencode
• bunx oxlint packages/opencode/src/provider/transform.ts packages/opencode/test/provider/transform.test.ts from repo root (existing warnings only)
• git diff --check
• pre-push bun turbo typecheck

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[rekram1-node]

minimax team explicitly requested this to be added, maybe they have a bug on their end but we want to enable thinking by default.