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.go

@@ -7,7 +7,9 @@ package util
 import (
 	"cmp"
 	"iter"
+	"reflect"
 	"slices"
+	"strings"
 )
 
 // Helpers below are copied from gopls' moremaps package.
@@ -34,3 +36,40 @@ func KeySlice[M ~map[K]V, K comparable, V any](m M) []K {
 	}
 	return r
 }
+
+type JSONInfo struct {
+	Omit     bool            // unexported or first tag element is "-"
+	Name     string          // Go field name or first tag element. Empty if Omit is true.
+	Settings map[string]bool // "omitempty", "omitzero", etc.
+}
+
+// FieldJSONInfo reports information about how encoding/json
+// handles the given struct field.
+// If the field is unexported, JSONInfo.Omit is true and no other JSONInfo field
+// is populated.
+// If the field is exported and has no tag, then Name is the field's name and all
+// other fields are false.
+// Otherwise, the information is obtained from the tag.
+func FieldJSONInfo(f reflect.StructField) JSONInfo {
+	if !f.IsExported() {
+		return JSONInfo{Omit: true}
+	}
+	info := JSONInfo{Name: f.Name}
+	if tag, ok := f.Tag.Lookup("json"); ok {
+		name, rest, found := strings.Cut(tag, ",")
+		// "-" means omit, but "-," means the name is "-"
+		if name == "-" && !found {
+			return JSONInfo{Omit: true}
+		}
+		if name != "" {
+			info.Name = name
+		}
+		if len(rest) > 0 {
+			info.Settings = map[string]bool{}
+			for _, s := range strings.Split(rest, ",") {
+				info.Settings[s] = true
+			}
+		}
+	}
+	return info
+}