fix(cli): Improve error message when adk web is run in wrong directory

Merge #3196

## Summary
Enhances the `AgentLoader` error message to provide clear guidance when users run `adk web` from incorrect directories.

## Motivation
During internal workshops, multiple teams encountered confusion when starting `adk web` from the wrong directory. This often happened when:
- Running `adk web my_agent/` instead of `adk web .`
- Being inside an agent directory when executing the command
- Configuring incorrect start paths during development

## Changes
- **Smart detection**: Checks if `agent.py` or `root_agent.yaml` exists in the current directory
- **Visual diagram**: Shows expected directory structure with actual agent name
- **Explicit command**: Includes `adk web <agents_dir>` usage example
- **Conditional hint**: Provides targeted guidance when user is detected to be inside an agent directory

## Example Error Message

### Before
```
ValueError: No root_agent found for 'my_agent'. Searched in 'my_agent.agent.root_agent', 'my_agent.root_agent' and 'my_agent/root_agent.yaml'. Ensure 'path/my_agent' is structured correctly, an .env file can be loaded if present, and a root_agent is exposed.
```

### After
```
ValueError: No root_agent found for 'my_agent'. Searched in 'my_agent.agent.root_agent', 'my_agent.root_agent' and 'my_agent/root_agent.yaml'.

Expected directory structure:
  <agents_dir>/
    my_agent/
      agent.py (with root_agent) OR
      root_agent.yaml

Then run: adk web <agents_dir>

Ensure 'path/my_agent' is structured correctly, an .env file can be loaded if present, and a root_agent is exposed.

HINT: It looks like you might be running 'adk web' from inside an agent directory. Try running 'adk web .' from the parent directory that contains your agent folder, not from within the agent folder itself.
```

## Testing
- ✅ Existing unit tests pass (17/22, with 5 pre-existing failures unrelated to this change)
- ✅ `test_agent_not_found_error` passes, confirming error message enhancement works correctly
- ✅ Code follows ADK contribution guidelines

## Type
- [x] Bug fix (improved error messaging)
- [ ] Feature
- [ ] Breaking change
- [ ] Documentation

## Related
Fixes #3195

---

**Tags**: #non-breaking

🤖 Generated with [Claude Code](https://claude.com/claude-code)

COPYBARA_INTEGRATE_REVIEW=#3196 from jpantsjoha:fix/improve-adk-web-error-message a73b190
PiperOrigin-RevId: 822186700

Author: jpantsjoha

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

@@ -224,13 +224,30 @@ def _perform_load(self, agent_name: str) -> Union[BaseAgent, App]:
       return root_agent
 
     # If no root_agent was found by any pattern
+    # Check if user might be in the wrong directory
+    hint = ""
+    agents_path = Path(agents_dir)
+    if (
+        agents_path.joinpath("agent.py").is_file()
+        or agents_path.joinpath("root_agent.yaml").is_file()
+    ):
+      hint = (
+          "\n\nHINT: It looks like you might be running 'adk web' from inside"
+          " an agent directory. Try running 'adk web .' from the parent"
+          " directory that contains your agent folder, not from within the"
+          " agent folder itself."
+      )
+
     raise ValueError(
         f"No root_agent found for '{agent_name}'. Searched in"
         f" '{actual_agent_name}.agent.root_agent',"
         f" '{actual_agent_name}.root_agent' and"
-        f" '{actual_agent_name}/root_agent.yaml'. Ensure"
-        f" '{agents_dir}/{actual_agent_name}' is structured correctly, an .env"
-        " file can be loaded if present, and a root_agent is exposed."
+        f" '{actual_agent_name}/root_agent.yaml'.\n\nExpected directory"
+        f" structure:\n  <agents_dir>/\n    {actual_agent_name}/\n     "
+        " agent.py (with root_agent) OR\n      root_agent.yaml\n\nThen run:"
+        f" adk web <agents_dir>\n\nEnsure '{agents_dir}/{actual_agent_name}' is"
+        " structured correctly, an .env file can be loaded if present, and a"
+        f" root_agent is exposed.{hint}"
     )
 
   def _ensure_app_name_matches(