Add support for embedded structs in structdiff (#3759)

## Changes

This change adds proper handling of anonymous (embedded) struct fields
in the structdiff package. When comparing structs with embedded fields,
the embedded struct's fields now appear at the root level of the path
rather than being prefixed with the embedded struct's name.

For example, with:
```go
type Embedded struct {
    Field string `json:"field"`
}
type Container struct {
    Embedded
    Name string `json:"name"`
}
```

A change to `Field` will be reported as "field" instead of
"Embedded.field".

## Why
Dashboards have embedded strings in the resource definition. This is
needed so that specifying `parent_path` as a key works in field
triggers. Otherwise you would need to specify `Dashboard.parent_path` to
account for the embedded struct.

## Tests
Added comprehensive test coverage for:
- Single embedded field changes
- Multiple embedded field changes
- Mixed embedded and non-embedded field changes
- Zero to non-zero transitions with omitempty
- Non-zero to zero transitions

All test cases include mirror tests and self-comparison tests.

Author: shreyas-goenka

libs/structs/structdiff/diff.go

@@ -127,6 +127,12 @@ func diffStruct(path *structpath.PathNode, s1, s2 reflect.Value, changes *[]Chan
 			continue
 		}
 
+		// Continue traversing embedded structs. Do not add the key to the path though.
+		if sf.Anonymous {
+			diffValues(path, s1.Field(i), s2.Field(i), changes)
+			continue
+		}
+
 		jsonTag := structtag.JSONTag(sf.Tag.Get("json"))
 
 		// Resolve field name from JSON tag or fall back to Go field name

libs/structs/structdiff/diff_test.go

@@ -26,6 +26,21 @@ type C struct {
 	ForceSendFields []string `json:"-"`
 }
 
+type Embedded struct {
+	EmbeddedString string `json:"embedded_field,omitempty"`
+	EmbeddedInt    int    `json:"embedded_int,omitempty"`
+}
+
+type D struct {
+	Embedded        // Anonymous embedded struct
+	Name     string `json:"name,omitempty"`
+}
+
+type E struct {
+	*Embedded        // Pointer to anonymous embedded struct
+	Name      string `json:"name,omitempty"`
+}
+
 // ResolvedChange represents a change with the field path as a string (like the old Change struct)
 type ResolvedChange struct {
 	Field string
@@ -260,6 +275,118 @@ func TestGetStructDiff(t *testing.T) {
 				{Field: "key1.is_enabled", Old: false, New: nil},
 			},
 		},
+
+		// Embedded struct tests
+		{
+			name: "embedded struct field change",
+			a:    D{Embedded: Embedded{EmbeddedString: "old"}, Name: "test"},
+			b:    D{Embedded: Embedded{EmbeddedString: "new"}, Name: "test"},
+			want: []ResolvedChange{{Field: "embedded_field", Old: "old", New: "new"}},
+		},
+		{
+			name: "embedded struct multiple field changes",
+			a:    D{Embedded: Embedded{EmbeddedString: "old", EmbeddedInt: 5}, Name: "test"},
+			b:    D{Embedded: Embedded{EmbeddedString: "new", EmbeddedInt: 10}, Name: "test"},
+			want: []ResolvedChange{
+				{Field: "embedded_field", Old: "old", New: "new"},
+				{Field: "embedded_int", Old: 5, New: 10},
+			},
+		},
+		{
+			name: "embedded and non-embedded field changes",
+			a:    D{Embedded: Embedded{EmbeddedString: "old"}, Name: "alice"},
+			b:    D{Embedded: Embedded{EmbeddedString: "new"}, Name: "bob"},
+			want: []ResolvedChange{
+				{Field: "embedded_field", Old: "old", New: "new"},
+				{Field: "name", Old: "alice", New: "bob"},
+			},
+		},
+		{
+			name: "embedded struct zero to non-zero",
+			a:    D{Name: "test"},
+			b:    D{Embedded: Embedded{EmbeddedString: "value"}, Name: "test"},
+			want: []ResolvedChange{{Field: "embedded_field", Old: nil, New: "value"}},
+		},
+		{
+			name: "embedded struct non-zero to zero",
+			a:    D{Embedded: Embedded{EmbeddedString: "value"}, Name: "test"},
+			b:    D{Name: "test"},
+			want: []ResolvedChange{{Field: "embedded_field", Old: "value", New: nil}},
+		},
+		{
+			name: "embedded struct both zero",
+			a:    D{Name: "test"},
+			b:    D{Name: "test"},
+			want: nil,
+		},
+		{
+			name: "embedded struct only non-embedded field changes",
+			a:    D{Embedded: Embedded{EmbeddedString: "same"}, Name: "alice"},
+			b:    D{Embedded: Embedded{EmbeddedString: "same"}, Name: "bob"},
+			want: []ResolvedChange{{Field: "name", Old: "alice", New: "bob"}},
+		},
+
+		// Pointer embedded struct tests
+		{
+			name: "pointer embedded struct field change",
+			a:    E{Embedded: &Embedded{EmbeddedString: "old"}, Name: "test"},
+			b:    E{Embedded: &Embedded{EmbeddedString: "new"}, Name: "test"},
+			want: []ResolvedChange{{Field: "embedded_field", Old: "old", New: "new"}},
+		},
+		{
+			name: "pointer embedded struct multiple field changes",
+			a:    E{Embedded: &Embedded{EmbeddedString: "old", EmbeddedInt: 5}, Name: "test"},
+			b:    E{Embedded: &Embedded{EmbeddedString: "new", EmbeddedInt: 10}, Name: "test"},
+			want: []ResolvedChange{
+				{Field: "embedded_field", Old: "old", New: "new"},
+				{Field: "embedded_int", Old: 5, New: 10},
+			},
+		},
+		{
+			name: "pointer embedded and non-embedded field changes",
+			a:    E{Embedded: &Embedded{EmbeddedString: "old"}, Name: "alice"},
+			b:    E{Embedded: &Embedded{EmbeddedString: "new"}, Name: "bob"},
+			want: []ResolvedChange{
+				{Field: "embedded_field", Old: "old", New: "new"},
+				{Field: "name", Old: "alice", New: "bob"},
+			},
+		},
+		{
+			name: "pointer embedded struct nil to non-nil",
+			a:    E{Name: "test"},
+			b:    E{Embedded: &Embedded{EmbeddedString: "value"}, Name: "test"},
+			want: []ResolvedChange{{Field: "", Old: (*Embedded)(nil), New: &Embedded{EmbeddedString: "value"}}},
+		},
+		{
+			name: "pointer embedded struct non-nil to nil",
+			a:    E{Embedded: &Embedded{EmbeddedString: "value"}, Name: "test"},
+			b:    E{Name: "test"},
+			want: []ResolvedChange{{Field: "", Old: &Embedded{EmbeddedString: "value"}, New: (*Embedded)(nil)}},
+		},
+		{
+			name: "pointer embedded struct both nil",
+			a:    E{Name: "test"},
+			b:    E{Name: "test"},
+			want: nil,
+		},
+		{
+			name: "pointer embedded struct only non-embedded field changes",
+			a:    E{Embedded: &Embedded{EmbeddedString: "same"}, Name: "alice"},
+			b:    E{Embedded: &Embedded{EmbeddedString: "same"}, Name: "bob"},
+			want: []ResolvedChange{{Field: "name", Old: "alice", New: "bob"}},
+		},
+		{
+			name: "pointer embedded struct zero to non-zero int",
+			a:    E{Name: "test"},
+			b:    E{Embedded: &Embedded{EmbeddedInt: 42}, Name: "test"},
+			want: []ResolvedChange{{Field: "", Old: (*Embedded)(nil), New: &Embedded{EmbeddedInt: 42}}},
+		},
+		{
+			name: "pointer embedded struct non-zero to zero int",
+			a:    E{Embedded: &Embedded{EmbeddedInt: 42}, Name: "test"},
+			b:    E{Name: "test"},
+			want: []ResolvedChange{{Field: "", Old: &Embedded{EmbeddedInt: 42}, New: (*Embedded)(nil)}},
+		},
 	}
 
 	for _, tt := range tests {