fix(mcp): emit portable schema version metadata

[Fail-Safe]

Summary

• emit schema_version as a portable non-negative integer schema without a client-specific uint32 format
• preserve the runtime u32 type and add regression coverage for the generated schema
• bump the package version to 0.21.1

Validation

• make test
• make release-check
• isolated OpenCode 1.18.19 MCP discovery against the optimized v0.21.1 binary; connected without schema warnings