chore: Adds indent to YAML sequence by default for better visual readability

Both are valid YAML, just with indent, it's more visually friend to see the data structure hierarchy.

Before

```
items:
- item1
- item2
- item3
```

After

```
items:
  - item1
  - item2
  - item3
```
PiperOrigin-RevId: 806117290

Author: Jacksunwei

src/google/adk/utils/yaml_utils.py

@@ -43,9 +43,16 @@ def dump_pydantic_to_yaml(
   file_path = Path(file_path)
   file_path.parent.mkdir(parents=True, exist_ok=True)
 
-  # Create a custom dumper class
   class _MultilineDumper(yaml.SafeDumper):
-    pass
+
+    def increase_indent(self, flow=False, indentless=False):
+      """Override to force consistent indentation for sequences in mappings.
+
+      By default, PyYAML uses indentless=True for sequences that are values
+      in mappings, creating flush-left alignment. This override forces proper
+      indentation for all sequences regardless of context.
+      """
+      return super(_MultilineDumper, self).increase_indent(flow, False)
 
   def multiline_str_representer(dumper, data):
     if '\n' in data:

tests/unittests/utils/test_yaml_utils.py

@@ -30,6 +30,7 @@ class SimpleModel(BaseModel):
   active: bool
   finish_reason: Optional[types.FinishReason] = None
   multiline_text: Optional[str] = None
+  items: Optional[list[str]] = None
 
 
 def test_yaml_file_generation(tmp_path: Path):
@@ -77,3 +78,48 @@ def test_multiline_string_pipe_style(tmp_path: Path):
   and should be formatted with pipe style
 name: Test
 """
+
+
+def test_list_indentation(tmp_path: Path):
+  """Test that lists in mappings are properly indented."""
+  model = SimpleModel(
+      name="Test",
+      age=25,
+      active=True,
+      items=["item1", "item2", "item3"],
+  )
+  yaml_file = tmp_path / "test.yaml"
+
+  dump_pydantic_to_yaml(model, yaml_file)
+
+  expected = """\
+active: true
+age: 25
+items:
+  - item1
+  - item2
+  - item3
+name: Test
+"""
+  assert yaml_file.read_text(encoding="utf-8") == expected
+
+
+def test_empty_list_formatting(tmp_path: Path):
+  """Test that empty lists are formatted properly."""
+  model = SimpleModel(
+      name="Test",
+      age=25,
+      active=True,
+      items=[],
+  )
+  yaml_file = tmp_path / "test.yaml"
+
+  dump_pydantic_to_yaml(model, yaml_file)
+
+  expected = """\
+active: true
+age: 25
+items: []
+name: Test
+"""
+  assert yaml_file.read_text(encoding="utf-8") == expected