## Why
Native `openai/userVerification` requests need to carry optional `_meta` to clients for extension-specific presentation.
## What changed
- Accept optional metadata objects with the same validation as MCP elicitation.
- Restore metadata from the MCP request context, excluding `progressToken`, and forward it through `mcpServer/elicitation/request`.
- Update protocol types, schemas, and documentation while retaining support for requests without metadata. Challenge bytes and acceptance proofs remain unchanged.
## Testing
Add coverage for metadata validation, transport context restoration, and protocol round trips. Extend MCP integration tests to check that verification metadata reaches the client.
GitOrigin-RevId: 5d04004244cf33effac675c19e543157c48d1504