feat: add MCP resource_link (2025-06-18)

[aharvard]

Align rmcp content models with MCP 2025-06-18 by adding the ResourceLink content block.

What’s included

• RawContent::ResourceLink (serde tag: resource_link)
• PromptMessageContent::ResourceLink for prompts/get
• Constructors: RawContent::resource_link and Content::resource_link
• Embedded text uses text/plain
• CamelCase (mimeType) compliance throughout
• Unit tests for resource_link and embedded text

Why

• Bring rmcp up to date with the latest MCP schema (2025-06-18) so tools and prompts can reference resources without embedding payloads.

Spec references (2025-06-18)

• ContentBlock includes: TextContent | ImageContent | AudioContent | ResourceLink | EmbeddedResource
• ResourceLink extends Resource (uri, name, description?, mimeType?, size?, annotations?)
• Schema: https://raw.githubusercontent.com/modelcontextprotocol/modelcontextprotocol/main/schema/2025-06-18/schema.ts

Notes

• rmcp represents ResourceLink as a flattened Annotated with type="resource_link"; annotations are carried by the Annotated wrapper (matching the spec’s Resource.annotations?).
• Title (BaseMetadata.title) is optional in the spec; rmcp currently uses name and optional annotations and does not add a separate title field.
• This PR only covers model/serde updates and tests; UI/host rendering for resource_link is out of scope.

[Copilot]

Pull Request Overview

This PR adds support for the MCP ResourceLink content block to align with the MCP schema specification dated 2025-06-18. The ResourceLink allows tools and prompts to reference server resources without embedding their full payloads, enabling on-demand fetching.

Key changes include:

• Addition of RawContent::ResourceLink and PromptMessageContent::ResourceLink variants with serde tag "resource_link"
• Helper constructors for creating resource link content blocks
• JSON schema updates reflecting the new content type structure
• Updated embedded text to use "text/plain" MIME type for compliance

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

File	Description
crates/rmcp/src/model/content.rs	Adds ResourceLink variant to RawContent enum and constructor methods
crates/rmcp/src/model/prompt.rs	Adds ResourceLink variant to PromptMessageContent enum and constructor
crates/rmcp/tests/test_message_schema/*.json	Updates JSON schemas to include new ResourceLink definitions and structure

Comments suppressed due to low confidence (1)

crates/rmcp/src/model/content.rs:1

• This test function is placed outside the mod tests block. It should be moved inside the existing test module to maintain consistent test organization.

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