LongRunningFunctionTool: no way to skip LLM summarization when client sends back function_response

## Is your feature request related to a specific problem?

Yes, when using `LongRunningFunctionTool` for Human-in-the-Loop (HITL) flows, the client eventually sends a `function_response` back to the agent (e.g., "user approved" / "user denied"). ADK passes this `function_response` to the LLM, which then generates an unnecessary text summary like "Understood." or "Done." in my case but it could by anything else.

There is no clean way to prevent this text generation:

- **`tool_context.actions.skip_summarization`** only fires during the initial tool execution (`after_tool_callback`), not when the client's `function_response` arrives in a subsequent invocation.
- **`before_model_callback`** can intercept the LLM call and return an empty `LlmResponse`, but `LlmResponse` has no `actions` field. So `skip_summarization` cannot be set. The empty `LlmResponse(content=Content(role="model", parts=[Part(text="")]))` gets persisted to session history, causing the LLM to generate acknowledgment text ("Understood.", "Done.") on the *next* user message.

This is especially problematic on **Agent Engine** (`VertexAiSessionService`), where the built-in HITL tools (`adk_request_confirmation`, `adk_request_input`) are explicitly **not supported**, making `LongRunningFunctionTool` the only available mechanism for HITL flows.

## Describe the Solution You'd Like

I think any of the following would solve the problem:

**1. A `skip_summarization_on_response` flag on `LongRunningFunctionTool`**

```python
send_email_tool = LongRunningFunctionTool(
func=send_campaign_email,
skip_summarization_on_response=True, # Skip LLM call when client responds
)
```

When the client sends back a `function_response` for this tool, ADK skips the LLM summarization call entirely and does not persist an empty model turn to session history.

**2. Allow `before_model_callback` to return `Event` (not just `LlmResponse`)**

This would let the callback set `actions.skip_summarization = True`, giving the developer full control:

```python
def my_callback(callback_context, llm_request):
# ... detect HITL function_response ...
return Event(
actions=EventActions(skip_summarization=True),
content=Content(role="model", parts=[]),
)
```

**3. An `on_function_response_callback`** that fires when a client sends back a `function_response` for a `LongRunningFunctionTool`, giving access to `ToolContext` with `actions.skip_summarization`.

I think option 1 is the simplest and most targeted.

## Impact on your work

This affects anyone deploying custom HITL flows on Agent Engine. Since `VertexAiSessionService` doesn't support the built-in confirmation tools, `LongRunningFunctionTool` is the only option. But it generates unwanted LLM text after every approval/denial. The current workaround (returning an empty `LlmResponse` via `before_model_callback`) works but is a workaround, not a proper solution.

## Willingness to contribute

Yes, I would be very happy to submit a PR if the team agrees on an approach.

---

## Describe Alternatives You've Considered

**Current workaround:** A `before_model_callback` that returns an empty `LlmResponse` when it detects a `function_response` from a HITL tool, skipping the LLM call entirely:

```python
_HITL_TOOL_NAMES = {"send_campaign_email"}

def _skip_hitl_summarization(callback_context, llm_request):
contents = llm_request.contents or []
if not contents:
return None

for part in contents[-1].parts or []:
if part.function_response and part.function_response.name in _HITL_TOOL_NAMES:
return LlmResponse(
content=types.Content(role="model", parts=[types.Part(text="")])
)

return None
```

This prevents the LLM from generating text on the current turn, but the empty model response still gets persisted to session history. A first-class `skip_summarization` mechanism would avoid this entirely.

## Additional Context

- ADK version: latest (`pip install google-adk`)
- Deployment: Vertex AI Agent Engine (`VertexAiSessionService`)
- The built-in HITL tools (`adk_request_confirmation`, `adk_request_input`) explicitly list `VertexAiSessionService` as unsupported in the [ADK docs](https://google.github.io/adk-docs/tools-custom/confirmation/#known-limitations), making `LongRunningFunctionTool` the only viable HITL mechanism on Agent Engine.
- Related issues: [#760](https://github.com/google/adk-python/issues/760), [#173](https://github.com/google/adk-python/issues/173), [#302](https://github.com/google/adk-python/issues/302), [#3986](https://github.com/google/adk-python/issues/3986)