mcp: update JSON marshaling to not HTML-escape messages (#769)

It's causing some servers to fail: #767. I checked with Typescript
implementation and it doesn't do any escaping.

It's still a behavior change, so we'll expose a way to bring back the previous behavior via an env var.

Fixes #238 
Fixes #767

internal/mcpgodebug/mcpgodebug_test.go

@@ -0,0 +1,88 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+package mcpgodebug
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestParseCompatibility_Success(t *testing.T) {
+	tests := []struct {
+		name   string
+		envVal string
+		want   map[string]string
+	}{
+		{
+			name:   "Basic",
+			envVal: "foo=bar,baz=qux",
+			want: map[string]string{
+				"foo": "bar",
+				"baz": "qux",
+			},
+		},
+		{
+			name:   "Empty",
+			envVal: "",
+			want:   nil,
+		},
+		{
+			name:   "WithWhitespace",
+			envVal: "  foo = bar  \t,  baz  = qux  ",
+			want: map[string]string{
+				"foo": "bar",
+				"baz": "qux",
+			},
+		},
+		{
+			name:   "WithEqualsSignInValue",
+			envVal: "foo=bar=baz",
+			want: map[string]string{
+				"foo": "bar=baz",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := parseCompatibility(tt.envVal)
+			if err != nil {
+				t.Fatalf("parseCompatibility() failed: %v", err)
+			}
+			if diff := cmp.Diff(tt.want, got); diff != "" {
+				t.Errorf("parseCompatibility() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+func TestParseCompatibility_Failure(t *testing.T) {
+	tests := []struct {
+		name   string
+		envVal string
+	}{
+		{
+			name:   "NoEqualsSign",
+			envVal: "invalidformat",
+		},
+		{
+			name:   "MixedValidAndInvalid",
+			envVal: "foo=bar,baz",
+		},
+		{
+			name:   "EmptyPart",
+			envVal: "foo=bar,,baz=qux",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := parseCompatibility(tt.envVal)
+			if err == nil {
+				t.Error("parseCompatibility() expected error, got nil")
+			}
+		})
+	}
+}