feat(rmcp): add optional _meta to CallToolResult, EmbeddedResource, and ResourceContents

[aharvard]

Summary

Support the optional _meta field across core MCP results and resources so protocol-level metadata can be exchanged and properly serialized/deserialized per the MCP spec.

Problem

The MCP spec allows an optional _meta field on results and certain content forms. The Rust SDK only partially supported this, limiting interoperability (e.g., clients expecting _meta could not round-trip metadata).

Changes

• CallToolResult
  • Add optional _meta (serde rename to "_meta", skip when None)
  • Custom deserializer accepts incoming "_meta"
• EmbeddedResource
  • RawEmbeddedResource now supports optional _meta (serde rename to "_meta")
• ResourceContents
  • TextResourceContents and BlobResourceContents now support optional _meta (serde rename to "_meta")
• Meta
  • Derive PartialEq and schemars::JsonSchema for testing and schema generation
• Schemas
  • Update JSON schema snapshots to reflect the new optional fields

Tests

• Added
  • crates/rmcp/tests/test_tool_result_meta.rs: serialize/deserialize CallToolResult with and without _meta
  • crates/rmcp/tests/test_embedded_resource_meta.rs: serialize/deserialize EmbeddedResource and inner Text/Blob contents with and without _meta
• Updated
  • tests/test_message_schema/client_json_rpc_message_schema.json
  • tests/test_message_schema/server_json_rpc_message_schema.json

Backward compatibility

No breaking changes: all new fields are optional and omitted when None.

Examples

CallToolResult:

{
  "content": [{"type":"text","text":"ok"}],
  "_meta": {"foo":"bar"}
}

EmbeddedResource with Text contents:

{
  "type": "resource",
  "_meta": {"top": 1},
  "resource": {
    "uri": "str://example",
    "mimeType": "text/plain",
    "text": "hello",
    "_meta": {"inner": 2}
  }
}

Notes

• Removed references to success_with_meta() / error_with_meta() helpers (not included).
• Python integration tests may fail locally; unit and schema tests pass.

[Kvadratni]

LGTM

[aharvard]

i need to clean up the test fixtures. not quite ready for final review.

[Kvadratni]

LGTM!

[alexhancock]

Let's follow up to make sure we're not missing it on any other required types but this LGTM for these types!

[aharvard]

Let's follow up to make sure we're not missing it on any other required types but this LGTM for these types!

sounds good, doing an analysis now