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.

[aharvard]

FYI... I used the copilot commit suggestion and it introduce a failed merge check due to git message formatting

[Copilot]

Pull Request Overview

This PR adds MCP ResourceLink content block support to align rmcp with the MCP 2025-06-18 specification, enabling references to server resources without embedding their payloads.

• Adds ResourceLink content type with URI-based resource referencing
• Updates embedded text MIME type to use proper "text/plain" instead of generic "text"
• Includes comprehensive test coverage for new functionality

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 constructors for resource linking
crates/rmcp/src/model/prompt.rs	Adds ResourceLink support to PromptMessageContent for prompts/get
crates/rmcp/tests/test_message_schema/*.json	Updates JSON schema files to reflect new ResourceLink type definitions

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[Copilot]

The comment refers to 'MCP draft schema' but the PR description indicates this aligns with MCP 2025-06-18. Consider updating to specify the exact schema version for clarity.

Suggested change

	//! NOTE: This file models MCP ContentBlock union types. Keep in sync with MCP draft schema.
	//! NOTE: This file models MCP ContentBlock union types. Keep in sync with MCP 2025-06-18 schema.