mcp: refactor streamable serving and support JSON responses

WIP
DO NOT REVIEW
DO NOT SUBMIT

Fixes #211

Author: findleyr

mcp/client.go

@@ -134,8 +134,8 @@ func (c *Client) Connect(ctx context.Context, t Transport) (cs *ClientSession, e
 		return nil, unsupportedProtocolVersionError{res.ProtocolVersion}
 	}
 	cs.initializeResult = res
-	if hc, ok := cs.mcpConn.(httpConnection); ok {
-		hc.setProtocolVersion(res.ProtocolVersion)
+	if hc, ok := cs.mcpConn.(clientConnection); ok {
+		hc.initialized(res)
 	}
 	if err := handleNotify(ctx, cs, notificationInitialized, &InitializedParams{}); err != nil {
 		_ = cs.Close()
@@ -250,7 +250,7 @@ func (c *Client) listRoots(_ context.Context, _ *ClientSession, _ *ListRootsPara
 func (c *Client) createMessage(ctx context.Context, cs *ClientSession, params *CreateMessageParams) (*CreateMessageResult, error) {
 	if c.opts.CreateMessageHandler == nil {
 		// TODO: wrap or annotate this error? Pick a standard code?
-		return nil, &jsonrpc2.WireError{Code: CodeUnsupportedMethod, Message: "client does not support CreateMessage"}
+		return nil, jsonrpc2.NewError(codeUnsupportedMethod, "client does not support CreateMessage")
 	}
 	return c.opts.CreateMessageHandler(ctx, cs, params)
 }

mcp/mcp_test.go

@@ -281,7 +281,7 @@ func TestEndToEnd(t *testing.T) {
 		} {
 			rres, err := cs.ReadResource(ctx, &ReadResourceParams{URI: tt.uri})
 			if err != nil {
-				if code := errorCode(err); code == CodeResourceNotFound {
+				if code := errorCode(err); code == codeResourceNotFound {
 					if tt.mimeType != "" {
 						t.Errorf("%s: not found but expected it to be", tt.uri)
 					}

mcp/resource.go

@@ -41,7 +41,7 @@ type ResourceHandler func(context.Context, *ServerSession, *ReadResourceParams)
 // not be found.
 func ResourceNotFoundError(uri string) error {
 	return &jsonrpc2.WireError{
-		Code:    CodeResourceNotFound,
+		Code:    codeResourceNotFound,
 		Message: "Resource not found",
 		Data:    json.RawMessage(fmt.Sprintf(`{"uri":%q}`, uri)),
 	}

mcp/shared.go

@@ -275,9 +275,9 @@ func sessionMethod[S Session, P Params, R Result](f func(S, context.Context, P)
 
 // Error codes
 const (
-	CodeResourceNotFound = -32002
+	codeResourceNotFound = -32002
 	// The error code if the method exists and was called properly, but the peer does not support it.
-	CodeUnsupportedMethod = -31001
+	codeUnsupportedMethod = -31001
 )
 
 func callNotificationHandler[S Session, P any](ctx context.Context, h func(context.Context, S, *P), sess S, params *P) (Result, error) {