[#67] Fix Client's ReadResource method

[rvoh-emccaleb]

Pre-requisite

Review/merge #66, first.

Issue

#67

Explanation of bug

The Client's current ReadResource() method does not correctly parse the response from the Protocol's request to the Server.

That response, returned by the Server, is ultimately enforced to be of type *ResourceResponse when the resource is registered at server startup. This type differs from what the Client is currently written to expect.

Solution

Updated the Client's ReadResponse() method to expect type *ResourceResponse.

I can now set up the server, locally, with:

err = server.RegisterResource(
	"example_resource.txt",
	"example",
	"Description for example resource",
	"text/plain",
	func() (*mcp.ResourceResponse, error) {
		content, err := os.ReadFile("internal/mcpserver/resources/example_resource.txt")
		if err != nil {
			return nil, fmt.Errorf("failed to read example resource: %w", err)
		}

		embeddedResource := mcp.NewTextEmbeddedResource("example_resource.txt", string(content), "text/plain")
		
		return mcp.NewResourceResponse(embeddedResource), nil
	},
)

And I can run the client, locally:

clientTransport := http.NewHTTPClientTransport("/mcp")
clientTransport.WithBaseURL("http://localhost:8080")
...
if _, err := client.Initialize(context.Background()); err != nil {
	log.Fatalf("failed to initialize client: %v", err)
}
...
exampleResponse, err := client.ReadResource(context.Background(), "example_resource.txt")
if err != nil {
	log.Fatalf("failed to read example resource: %v", err)
}

good := exampleResponse != nil && len(exampleResponse.Contents) > 0 && exampleResponse.Contents[0] != nil && exampleResponse.Contents[0].TextResourceContents != nil
if !good {
	log.Fatalf("invalid resource response received: %+v", exampleResponse)
}

log.Printf("example resource response: %s", exampleResponse.Contents[0].TextResourceContents.Text)

Running the client outputs:

example resource response: <contents-of-internal/mcpserver/resources/example_resource.txt>

[rvoh-emccaleb]

The changes in this file are written to satisfy the following constraint:

1. Don't break the existing marshaling/unmarshaling API for EmbeddedResource.
   1. Add an embeddedResourceType field (don't touch existing fields).
   2. Continue to allow the multiple content types and schemas (BlobResourceContents, TextResourceContents) over one endpoint.

[rvoh-emccaleb]

I don't like unmarshaling twice, but given the API supports multiple types/schemas that are identified in the body (not in headers), I don't currently see a way around it.

[rvoh-emccaleb]

Temporarily have to close this due to company OSS policies