* feat(rmcp): support optional _meta on EmbeddedResource and ResourceContents
- Add optional _meta to RawEmbeddedResource and ResourceContents (text/blob)
- Update constructors/usages to set meta: None by default
- Add tests to lock behavior (embedded text/blob + CallToolResult)
- Update JSON Schemas to include optional _meta fields
* style(rmcp): rustfmt after _meta changes and tests
