fix: filter non-agent directoris from list_agents()

Merge google#4648

**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) before creating a pull request.**

### Link to Issue or Description of Change

**1. Link to an existing issue (if applicable):**

- Closes: google#4647
- Related: google#3429, google#3430

**2. Or, if no issue exists, describe the change:**

**Problem:**
`AgentLoader.list_agents()` returns every non-hidden subdirectory in the agents directory, regardless of whether it contains a valid agent definition. This causes non-agent directories (e.g. `tmp/`, `data/`, `utils/`) to appear in the `/list-apps` API response. This affects both the ADK web UI agent selector and any production deployment depending on this API.

**Solution:**
Reuse the existing `_determine_agent_language()` method inside `list_agents()` to verify each candidate directory contains at least one recognized agent file (`root_agent.yaml`, `agent.py`, or `__init__.py`). Directories that fail this check are excluded from the result. This avoids introducing any new methods or abstractions and keeps the check lightweight (filesystem only, no agent imports).

### Testing Plan

**Unit Tests:**

- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.

27 passed in 2.85s:
pytest tests/unittests/cli/utils/test_agent_loader.py -v
======================= 27 passed, 14 warnings in 2.85s ========================

Added `test_list_agents_excludes_non_agent_directories` which creates a temp directory with three valid agent types (package with `__init__.py`, module with `agent.py`, YAML with `root_agent.yaml`) and three non-agent directories, and asserts only the valid agents are listed.

**Screenshots / Video:**
| Before (non-agent directories listed) | After (only valid agents listed) |
|----------------------------------------|----------------------------------|
|<img width="566" height="553" alt="Image" src="https://github.com/user-attachments/assets/0f50084b-319f-480e-8d8a-051c28d4a7e7" />|<img width="567" height="532" alt="Image" src="https://github.com/user-attachments/assets/52d3543f-4c4c-4ff3-a6dd-7d5ce3f19bb2" />|

**Manual End-to-End (E2E) Tests:**

1. Create a project directory containing both valid agent subdirectories and non-agent subdirectories
2. Run `adk web .`
3. Open the web UI and verify only valid agents appear in the agent selector
4. See screenshots below for before/after comparison

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [ ] Any dependent changes have been merged and published in downstream modules.

### Additional context

COPYBARA_INTEGRATE_REVIEW=google#4648 from markadelnawar:fix/list-agents-filter-non-agents-dirs 0418956
PiperOrigin-RevId: 878674609

Author: markadelnawar

src/google/adk/cli/utils/agent_loader.py

@@ -335,13 +335,18 @@ def load_agent(self, agent_name: str) -> Union[BaseAgent, App]:
   def list_agents(self) -> list[str]:
     """Lists all agents available in the agent loader (sorted alphabetically)."""
     base_path = Path.cwd() / self.agents_dir
-    agent_names = [
-        x
-        for x in os.listdir(base_path)
-        if os.path.isdir(os.path.join(base_path, x))
-        and not x.startswith(".")
-        and x != "__pycache__"
-    ]
+    agent_names = []
+    for x in os.listdir(base_path):
+      if (
+          os.path.isdir(os.path.join(base_path, x))
+          and not x.startswith(".")
+          and x != "__pycache__"
+      ):
+        try:
+          self._determine_agent_language(x)
+          agent_names.append(x)
+        except ValueError:
+          continue
     agent_names.sort()
     return agent_names
 

tests/unittests/cli/utils/test_agent_loader.py

@@ -993,3 +993,48 @@ def __init__(self):
       assert len(detailed_list) == 1
       assert detailed_list[0]["name"] == agent_name
       assert not detailed_list[0]["is_computer_use"]
+
+  def test_list_agents_excludes_non_agent_directories(self):
+    """Test that list_agents filters out directories without agent definitions."""
+    with tempfile.TemporaryDirectory() as temp_dir:
+      temp_path = Path(temp_dir)
+
+      valid_package = temp_path / "valid_agent"
+      valid_package.mkdir()
+      (valid_package / "__init__.py").write_text(dedent("""
+          from google.adk.agents.base_agent import BaseAgent
+
+          class ValidAgent(BaseAgent):
+              def __init__(self):
+                  super().__init__(name="valid_agent")
+
+          root_agent = ValidAgent()
+      """))
+
+      valid_module = temp_path / "module_agent"
+      valid_module.mkdir()
+      (valid_module / "agent.py").write_text(dedent("""
+          from google.adk.agents.base_agent import BaseAgent
+
+          class ModuleAgent(BaseAgent):
+              def __init__(self):
+                  super().__init__(name="module_agent")
+
+          root_agent = ModuleAgent()
+      """))
+
+      valid_yaml = temp_path / "yaml_agent"
+      valid_yaml.mkdir()
+      (valid_yaml / "root_agent.yaml").write_text("name: yaml_agent\n")
+
+      (temp_path / "random_folder").mkdir()
+      (temp_path / "data").mkdir()
+      (temp_path / "tmp").mkdir()
+
+      loader = AgentLoader(str(temp_path))
+      agents = loader.list_agents()
+
+      assert agents == ["module_agent", "valid_agent", "yaml_agent"]
+      assert "random_folder" not in agents
+      assert "data" not in agents
+      assert "tmp" not in agents