feat: add ResourceLink content type (MCP spec 2025-06-18)

[rupeshstha]

What

Adds the resource_link content type from the MCP specification (2025-06-18).

Closes #199

Why

resource_link is the only first-class content type in the spec not yet implemented. Unlike an embedded resource (which inlines content), a resource_link returns a URI pointer, the client fetches or subscribes independently. This matters for large payloads and live subscriptions.

Changes

• New src/Server/Content/ResourceLink.php — implements the Content contract with HasMeta, constructor accepts $uri (required) plus optional $name, $mimeType, $description; null fields are omitted from output
• Modified src/Response.php — adds Response::resourceLink() static factory, consistent with text(), image(), audio()
• New tests/Unit/Content/ResourceLinkTest.php 12 tests, 40 assertions covering all fields, null omission, _meta, __toString(), and delegation to toTool()/toPrompt()/toResource()

Modified: src/Response.php

• Imported use Laravel\Mcp\Server\Content\ResourceLink; import.
  Add ResourceLink Likewise Added Response::resourceLink() static factory method - same as existing text, image, audio methods.

New: tests/Unit/Content/ResourceLinkTest.php

• 12 tests, 40 assertions
• Covers: all optional fields, null field omission, support of the meta, delegation of toTool() / toPrompt() / toResource(), support of __toString().

---

Usage

use Laravel\Mcp\Response;

// In a Tool handler
public function handle(): array
{
    return [
        Response::resourceLink(
            uri: 'file:///data/report.json',
            name: 'Monthly Report',
            mimeType: 'application/json',
            description: 'Generated sales report',
        ),
    ];
}

Result (spec-compliant):

{
  "type": "resource_link",
  "uri": "file:///data/report.json",
  "name": "Monthly Report",
  "mimeType": "application/json",
  "description": "Generated sales report."
}