fix: strip incompatible thinking blocks when switching to Claude

[coleleavitt]

Fixes #6418

Problem

When switching from models like GLM 4.7 or MiniMax to Claude with extended thinking enabled, users get API errors:

messages.X.content.0: Invalid 'signature' in 'thinking' block

or

messages.X.content.0.type: Expected 'thinking' or 'redacted_thinking', but found 'text'

This happens because:

1. Other models produce thinking blocks with signatures that are incompatible with Claude's signature validation
2. Claude requires the last assistant message to start with a valid thinking block when thinking is enabled
3. reasoning parts from other models don't have the signature format Claude expects

Solution

In normalizeMessages(), when the target model is Claude:

1. Convert thinking blocks with signatures to wrapped text - Preserves the reasoning content while removing the invalid signature. Wrapped in <assistant_thinking> tags.
2. Convert reasoning parts to text - Same approach, wrapped in <assistant_reasoning> tags.
3. Track which messages had thinking blocks converted - Using a Set to avoid incorrectly removing messages that we already handled.
4. Only remove last assistant message if safe - Only when it doesn't start with valid thinking, has no tool calls, AND didn't have converted thinking blocks.

This approach preserves tool_use/tool_result pairing while still handling the signature validation issues.

Changes

• packages/opencode/src/provider/transform.ts: Added thinking block conversion logic in Claude-specific section
• packages/opencode/test/provider/thinking-blocks.test.ts: New test file with comprehensive coverage
• packages/opencode/test/provider/transform.test.ts: Updated existing test expectation

Testing

1. Start a session with GLM 4.7 or MiniMax (models that generate thinking/reasoning)
2. Send a few messages, including one that triggers tool use
3. Switch to Claude with extended thinking enabled
4. Send a message - should no longer get signature errors
5. Tool results should still work correctly

Related

• Supersedes #6748 (similar approach but that PR has been stale)
• Related to #7779 (GLM 4.7 thinking format issues)