internal/mcp: unify json tag parsing

Do a thorough job of parsing json struct tags.

Move the code to a place where both mcp and jsonschema can use it.

Change-Id: I6c86308834706b12a1e22cd96ae120590f49d3f4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/678757
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/util/util_test.go

@@ -0,0 +1,38 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package util
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestJSONInfo(t *testing.T) {
+	type S struct {
+		A int
+		B int `json:","`
+		C int `json:"-"`
+		D int `json:"-,"`
+		E int `json:"echo"`
+		F int `json:"foxtrot,omitempty"`
+		g int `json:"golf"`
+	}
+	want := []JSONInfo{
+		{Name: "A"},
+		{Name: "B"},
+		{Omit: true},
+		{Name: "-"},
+		{Name: "echo"},
+		{Name: "foxtrot", Settings: map[string]bool{"omitempty": true}},
+		{Omit: true},
+	}
+	tt := reflect.TypeFor[S]()
+	for i := range tt.NumField() {
+		got := FieldJSONInfo(tt.Field(i))
+		if !reflect.DeepEqual(got, want[i]) {
+			t.Errorf("got %+v, want %+v", got, want[i])
+		}
+	}
+}