fix: Handle App objects in eval and graph endpoints

Merge #3060

## Description

Fixes #3059

This PR fixes two endpoints in `adk web` that fail when using App objects instead of bare agents.

## Changes

- **Eval execution endpoint** (line ~969): Extract root_agent from App objects before passing to LocalEvalService
- **Graph visualization endpoint** (line ~1308): Extract root_agent from App objects before graph operations

Both endpoints now properly handle both BaseAgent and App objects by checking the type and extracting `.root_agent` when needed.

## Testing Plan

### Manual E2E Testing with ADK Web

Tested with an App object that includes context caching:

```python
from google.adk.apps import App
from google.adk.agents import LlmAgent

root_agent = LlmAgent(name="MyAgent", model="gemini-1.5-pro-002")
app = App(
    name="my_agent",
    root_agent=root_agent,
    context_cache_config=ContextCacheConfig(...)
)
```

**Before fix:**
- Graph visualization failed (tried to call agent methods on App object)
- Eval execution failed (LocalEvalService received App instead of agent)

**After fix:**
- Graph visualization works correctly
- Eval execution works correctly
- Both endpoints properly extract root_agent from App objects

## Checklist

- [x] Code follows project style (autoformat.sh passed)
- [x] Changes are focused and minimal
- [x] Issue #3059 created and referenced
- [x] Manual E2E testing completed

COPYBARA_INTEGRATE_REVIEW=#3060 from ejfn:ejfn/bugfix-app-object-endpoints 01c3019
PiperOrigin-RevId: 821746910

Author: ejfn

src/google/adk/cli/adk_web_server.py

@@ -486,6 +486,12 @@ async def get_runner_async(self, app_name: str) -> Runner:
     self.runner_dict[app_name] = runner
     return runner
 
+  def _get_root_agent(self, agent_or_app: BaseAgent | App) -> BaseAgent:
+    """Extract root agent from either a BaseAgent or App object."""
+    if isinstance(agent_or_app, App):
+      return agent_or_app.root_agent
+    return agent_or_app
+
   def _create_runner(self, agentic_app: App) -> Runner:
     """Create a runner with common services."""
     return Runner(
@@ -933,9 +939,8 @@ async def add_session_to_eval_set(
 
       # Populate the session with initial session state.
       agent_or_app = self.agent_loader.load_agent(app_name)
-      if isinstance(agent_or_app, App):
-        agent_or_app = agent_or_app.root_agent
-      initial_session_state = create_empty_state(agent_or_app)
+      root_agent = self._get_root_agent(agent_or_app)
+      initial_session_state = create_empty_state(root_agent)
 
       new_eval_case = EvalCase(
           eval_id=req.eval_id,
@@ -1096,7 +1101,8 @@ async def run_eval(
               status_code=400, detail=f"Eval set `{eval_set_id}` not found."
           )
 
-        root_agent = self.agent_loader.load_agent(app_name)
+        agent_or_app = self.agent_loader.load_agent(app_name)
+        root_agent = self._get_root_agent(agent_or_app)
 
         eval_case_results = []
 
@@ -1437,13 +1443,7 @@ async def get_event_graph(
       function_calls = event.get_function_calls()
       function_responses = event.get_function_responses()
       agent_or_app = self.agent_loader.load_agent(app_name)
-      # The loader may return an App; unwrap to its root agent so the graph builder
-      # receives a BaseAgent instance.
-      root_agent = (
-          agent_or_app.root_agent
-          if isinstance(agent_or_app, App)
-          else agent_or_app
-      )
+      root_agent = self._get_root_agent(agent_or_app)
       dot_graph = None
       if function_calls:
         function_call_highlights = []