structdiff: diff json:"-," fields like encoding/json does (#7207)

`GetStructDiff` skipped every field whose JSON name is `-`, including
`json:"-,"` / `json:"-,omitempty"` fields that encoding/json serializes
under the key `"-"`. It now skips only the exact tag `json:"-"` via
`structaccess.IsSkippedField`, which `IsEqual`, `structaccess` and
`structwalk` already use. Before this change `IsEqual` could report two
values as different while `GetStructDiff` returned no changes.

The change is reported at path `-`, the same form `structwalk` emits and
`structaccess.Get` resolves.

No type in this repo or in databricks-sdk-go uses such a tag (only test
fixtures do), so this is a consistency fix with no user-visible effect
today.

This pull request and its description were written by Isaac.

Co-authored-by: Isaac <no-reply@databricks.com>

Author: denik

libs/structs/structdiff/diff.go

@@ -219,13 +219,15 @@ func diffStruct(ctx *diffContext, path *structpath.PathNode, s1, s2 reflect.Valu
 			continue
 		}
 
+		// Only the exact tag json:"-" skips a field; json:"-," names it "-".
+		if structaccess.IsSkippedField(sf) {
+			continue
+		}
+
 		jsonTag := structtag.JSONTag(sf.Tag.Get("json"))
 
 		// Resolve field name from JSON tag or fall back to Go field name
 		fieldName := jsonTag.Name()
-		if fieldName == "-" {
-			continue
-		}
 
 		isEmbed := sf.Name == structaccess.EmbeddedSliceFieldName
 

libs/structs/structdiff/diff_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/databricks/cli/libs/structs/structaccess"
+
 	"github.com/databricks/databricks-sdk-go/common/types/duration"
 	sdktime "github.com/databricks/databricks-sdk-go/common/types/time"
 	"github.com/databricks/databricks-sdk-go/service/jobs"
@@ -1034,3 +1036,26 @@ func TestGetStructDiffWholeBlock(t *testing.T) {
 		})
 	}
 }
+
+type dashNamed struct {
+	Skipped string `json:"-"`
+	Named   string `json:"-,omitempty"` //nolint:staticcheck // odd tag is the point
+}
+
+// TestGetStructDiffDashNamedField checks that only the exact tag json:"-" skips a
+// field: encoding/json serializes json:"-,omitempty" under the key "-", and
+// structaccess resolves the path "-" to it.
+func TestGetStructDiffDashNamedField(t *testing.T) {
+	before := &dashNamed{Skipped: "a", Named: "a"}
+	after := &dashNamed{Skipped: "b", Named: "b"}
+
+	changes, err := GetStructDiff(before, after, nil)
+	require.NoError(t, err)
+	assert.Equal(t, []ResolvedChange{{Field: "-", Old: "a", New: "b"}}, resolveChanges(changes))
+
+	value, err := structaccess.Get(after, changes[0].Path)
+	require.NoError(t, err)
+	assert.Equal(t, "b", value)
+
+	assert.Equal(t, len(changes) == 0, IsEqual(before, after))
+}