Expose MCP read-only hints in tool call items

[copyberry]

Expose MCP read-only hints in tool call items

What changed

• Propagate MCP tool readOnlyHint annotations through tool-call start and
  completion events.
• Include the optional hint in persisted thread history and app-server
  mcpToolCall items, preserving compatibility when the annotation is absent.
• Document that the hint describes tool capability, not the outcome of a
  particular invocation.

Testing

• Cover read-only and write-capable tools in live events and persisted rollout
  data.
• Verify the hint survives thread reads and resumes for both in-progress and
  completed MCP calls.

[github-actions]

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