feat(v2): allow disabling Anthropic thinking block binding

[DEAN-Cherry]

Issue for this PR

Fixes #48757

Type of change

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

What does this PR do?

OpenCode V2 automatically adds Anthropic thinking block binding for Claude 5.1+ models. Some Anthropic-compatible endpoints reject thinking.block_binding entirely.

This exposes an opt-out in V2 and supports the V1-compatible model configuration:

{
  "provider": {
    "anthropic": {
      "models": {
        "claude-opus-5": {
          "options": {
            "thinking": {
              "type": "adaptive",
              "blockBinding": false
            }
          }
        }
      }
    }
  }
}

V2 consumes blockBinding: false before serializing the Anthropic request. The resulting request omits thinking.block_binding and the thinking-binding-controls-2026-08-01 beta header. When the option is absent, the existing model-ID-based default remains unchanged.

How did you verify your code works?

• AI Anthropic binding tests: 15 passed
• Schema model tests: 7 passed
• Core provider tests: 19 passed
• V1-to-V2 migration opt-out test passed
• AI, schema, and core typechecks passed

Screenshots / recordings

Not applicable; this is a non-UI change.

Checklist

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