internal/mcp: add more package documentation, examples

Add more documentation for the mcp package, explaining how to create and
use Clients and Servers. Also add an example of using the SSEHandler to
crete an MCP server.

Change-Id: Ib327826b2cdb1e3418482551c65569ec6f01af33
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668715
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

mcp/sse.go

@@ -277,14 +277,16 @@ type SSEClientTransport struct {
 
 // NewSSEClientTransport returns a new client transport that connects to the
 // SSE server at the provided URL.
-func NewSSEClientTransport(rawURL string) (*SSEClientTransport, error) {
-	url, err := url.Parse(rawURL)
+//
+// NewSSEClientTransport panics if the given URL is invalid.
+func NewSSEClientTransport(baseURL string) *SSEClientTransport {
+	url, err := url.Parse(baseURL)
 	if err != nil {
-		return nil, err
+		panic(fmt.Sprintf("invalid base url: %v", err))
 	}
 	return &SSEClientTransport{
 		sseEndpoint: url,
-	}, nil
+	}
 }
 
 // Connect connects through the client endpoint.