feat: add resource_link support to tools and prompts

[aharvard]

Summary

This PR adds support for the resource_link content type to the Rust SDK, as specified in the MCP specification. This allows both tools and prompts to return links to resources without embedding the full content, which is useful for large files or resources that should be fetched on-demand.

Changes

Core Implementation

• Added ResourceLink variant to RawContent enum in content.rs with snake_case discriminant
• Added ResourceLink variant to PromptMessageContent enum in prompt.rs
• Both enums use #[serde(rename_all = "snake_case")] to match the MCP spec

Helper Methods

• Content::resource_link() - Create resource link content for tools
• RawContent::resource_link() - Create raw resource link content
• RawContent::as_resource_link() - Access resource link from content
• PromptMessageContent::resource_link() - Create resource link for prompts
• PromptMessage::new_resource_link() - Create resource link prompt message

Tests

• Comprehensive unit tests for serialization/deserialization
• Integration tests verifying compatibility between tools and prompts
• Tests for mixed content types (text, resource links, embedded resources)
• All tests verify correct JSON format with "type": "resource_link"

Schema Updates

• Updated JSON schema generation to include the new resource_link type
• Schema fixtures updated to reflect the changes

Example Usage

Tools returning resource links:

use rmcp::model::{CallToolResult, Content, RawResource};

let resource = RawResource::new("file:///data.json", "Data File");
let result = CallToolResult::success(vec![
    Content::text("Found data file"),
    Content::resource_link(resource),
]);

Prompts returning resource links:

use rmcp::model::{PromptMessage, PromptMessageRole, RawResource};

let resource = RawResource::new("file:///doc.md", "Documentation");
let message = PromptMessage::new_resource_link(
    PromptMessageRole::User,
    resource.no_annotation(),
);

Compatibility

• Maintains backward compatibility - this is an additive change
• The mimeType field correctly uses camelCase in JSON serialization (Rust field is mime_type)
• Follows the MCP draft specification for resource links

Testing

All tests pass including:

• Unit tests for content types
• Integration tests for tool/prompt compatibility
• Schema generation tests
• Formatting and clippy checks

[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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[aharvard]

@4t145, I am converting this PR back to a draft state. Something happened when I rebased and force-pushed. I've been working thru failing merge checks with Goose and don't like the edits I'm seeing.

I'll notify when ready for another review.

[aharvard]

Ready for re-review @4t145. Please take another look at everything I re-rolled my attempt entirely.

Pinging @alexhancock as well.

[alexhancock]

Will look later today @aharvard