fix(samples): make a2a human-in-the-loop sample resume after confirmation

contributing/samples/a2a/a2a_human_in_loop/README.md

@@ -54,14 +54,14 @@ The A2A Human-in-the-Loop sample consists of:
 
    ```bash
    # Start the remote a2a server that serves the human-in-the-loop approval agent on port 8001
-   adk api_server --a2a --port 8001 contributing/samples/a2a_human_in_loop/remote_a2a
+   adk api_server --a2a --port 8001 contributing/samples/a2a/a2a_human_in_loop/remote_a2a
    ```
 
-1. **Run the Main Agent**:
+2. **Run the Main Agent**:
 
    ```bash
    # In a separate terminal, run the adk web server
-   adk web contributing/samples/
+   adk web contributing/samples/a2a
    ```
 
 ### Example Interactions
@@ -82,10 +82,24 @@ Agent: ✅ Reimbursement approved and processed: $50 for meals
 User: Please reimburse $200 for conference travel
 Agent: I'll process your reimbursement request for $200 for conference travel. Since this amount exceeds $100, I need to get manager approval.
 Agent: 🔄 Request submitted for approval (Ticket: reimbursement-ticket-001). Please wait for manager review.
-[Human manager interacts with root agent to approve the request]
+[Human manager approves the pending request from the ADK Web UI]
 Agent: ✅ Great news! Your reimbursement has been approved by the manager. Processing $200 for conference travel.
 ```
 
+> **Approving from the ADK Web UI:** The approval is a *long-running tool* call
+> that runs on the remote approval agent. The pending call is surfaced in the
+> Web UI as a function call awaiting a response. To approve (or reject), hover
+> over the pending `ask_for_approval` function response in the UI and use
+> **"Send another response"** to send back an updated response such as
+> `{"status": "approved", "ticketId": "reimbursement-ticket-001"}`. Simply
+> typing "I approve" as a chat message will **not** resume the pending request,
+> because the framework needs a `FunctionResponse` that carries the same call
+> `id` to resume the long-running tool.
+>
+> For this resume to be routed back to the remote approval agent (rather than
+> restarting at the root agent), the sample is exposed as an `App` with
+> `ResumabilityConfig(is_resumable=True)` in `agent.py`.
+
 ## Code Structure
 
 ### Main Agent (`agent.py`)

contributing/samples/a2a/a2a_human_in_loop/agent.py

@@ -16,6 +16,8 @@
 from google.adk.agents.llm_agent import Agent
 from google.adk.agents.remote_a2a_agent import AGENT_CARD_WELL_KNOWN_PATH
 from google.adk.agents.remote_a2a_agent import RemoteA2aAgent
+from google.adk.apps import App
+from google.adk.apps import ResumabilityConfig
 from google.genai import types
 
 
@@ -49,3 +51,18 @@ def reimburse(purpose: str, amount: float) -> str:
     sub_agents=[approval_agent],
     generate_content_config=types.GenerateContentConfig(temperature=0.1),
 )
+
+# The human-in-the-loop approval runs as a long-running tool on the remote
+# approval_agent. When the manager approves (or rejects) the request, the ADK
+# Web UI sends back a FunctionResponse for that pending long-running call. For
+# the next turn to be routed back to the (remote) approval_agent so it can
+# resume the paused tool instead of restarting at the root reimbursement_agent,
+# the app must be resumable. Without this, the confirmation is delivered to the
+# root agent, which has no pending call, and nothing happens (see issue #5871).
+app = App(
+    name='a2a_human_in_loop',
+    root_agent=root_agent,
+    resumability_config=ResumabilityConfig(
+        is_resumable=True,
+    ),
+)