fix: use toolConfirmation.payload as submit payload instead of tool args

[vietnamesekid]

Fixes #439

Problem

I was following the official docs at https://adk.dev/tools-custom/confirmation/#confirmation-definition
to implement a custom confirmation flow. When submitting the confirmation form, the agent
receives { days: 5 } (the tool's original args) instead of { approved_days: 0 } (the
payload schema defined by the developer), causing a KeyError crash.

Root Cause

In long-running-response.ts, initForm() initializes confirmationModel.payload from
originalFunctionCall.args instead of toolConfirmation.payload. Since onSend() reads
confirmationModel.payload to build the submit request, the wrong payload gets sent.

Fix

Read toolConfirmation.payload first when initializing confirmationModel.payload, with
a fallback to originalFunctionCall.args for cases where the developer did not provide a payload.

// before
this.confirmationModel.payload = JSON.stringify(
  this.functionCall.args?.originalFunctionCall?.args || {}, null, 2
);

// after
this.confirmationModel.payload = JSON.stringify(
  this.functionCall.args?.toolConfirmation?.payload ??
  this.functionCall.args?.originalFunctionCall?.args ??
  {}, null, 2
);

[vietnamesekid]

this.functionCall.args?.toolConfirmation?.payload is safe to read here because the backend always includes it when the developer provides a payload.

In adk-python/flows/llm_flows/functions.py, the adk_request_confirmation event is built with:

request_confirmation_function_call = types.FunctionCall(
    name=REQUEST_CONFIRMATION_FUNCTION_CALL_NAME,
    args={
        'originalFunctionCall': original_function_call.model_dump(...),
        'toolConfirmation': tool_confirmation.model_dump(by_alias=True, exclude_none=True),
    },
)

And ToolConfirmation is defined as:

class ToolConfirmation(BaseModel):
    hint: str = ""
    confirmed: bool = False
    payload: Optional[Any] = None

So when the developer calls tool_context.request_confirmation(payload={...}), payload is non-None and survives exclude_none=True, meaning toolConfirmation.payload will always be present in the event sent to the UI. The fallback to originalFunctionCall.args only kicks in when the developer did not provide a payload at all.

[vietnamesekid]

Tested with different payload schemas both {'approved_days': days} and {'days': days} work correctly after this fix. The submitted payload now always matches whatever schema the developer defines in request_confirmation(payload={...}) regardless of the key names