fix: Add signature field to ReasoningPart

[usvimal]

This PR adds the optional signature field to the ReasoningPart type to support thinking block signatures required by models like Claude.

Changes

• Added signature: z.string().optional() to ReasoningPart in packages/opencode/src/session/message-v2.ts
• Added signature: z.string().optional() to ReasoningPart in packages/opencode/src/session/message.ts
• Updated toModelMessage function to include the signature when converting reasoning parts

Related Issue

Fixes issue #6418: "Error: Invalid signature in thinking block" when switching models from MiniMax/GLM to Claude

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR fix: strip incompatible thinking blocks when switching to Anthropic models #6748: "fix: strip incompatible thinking blocks when switching to Anthropic models"
  • fix: strip incompatible thinking blocks when switching to Anthropic models #6748

Why they're related: Both PRs address compatibility issues when switching between different models and handling thinking/reasoning blocks. PR #6748 handles stripping incompatible thinking blocks when switching to Anthropic models, while PR #7222 adds signature field support for Claude. These changes may be related to the same issue (#6418) about model compatibility with thinking blocks.

[usvimal]

Closing as superseded by #6748 which provides a more complete fix - stripping incompatible thinking blocks and converting reasoning to text when switching to Anthropic models.