fix(backend): Error out execution when no starting node is available (#9566)

### Changes 🏗️

Prevent a graph being executed and do nothing by erroring out when no
starting node is available.

### Checklist 📋

#### For code changes:
- [ ] I have clearly listed my changes in the PR description
- [ ] I have made a test plan
- [ ] I have tested my changes according to the test plan:
  <!-- Put your test plan here: -->
  - [ ] ...

<details>
  <summary>Example test plan</summary>
  
  - [ ] Create from scratch and execute an agent with at least 3 blocks
- [ ] Import an agent from file upload, and confirm it executes
correctly
  - [ ] Upload agent to marketplace
- [ ] Import an agent from marketplace and confirm it executes correctly
  - [ ] Edit an agent from monitor, and confirm it executes correctly
</details>

#### For configuration changes:
- [ ] `.env.example` is updated or already compatible with my changes
- [ ] `docker-compose.yml` is updated or already compatible with my
changes
- [ ] I have included a list of my configuration changes in the PR
description (under **Changes**)

<details>
  <summary>Examples of configuration changes</summary>

  - Changing ports
  - Adding new services that need to communicate with each other
  - Secrets or environment variable changes
  - New or infrastructure changes such as databases
</details>

autogpt_platform/backend/backend/data/graph.py

@@ -365,10 +365,12 @@ class GraphModel(Graph):
                     )
                     and (
                         for_run  # Skip input completion validation, unless when executing.
-                        or block.block_type == BlockType.INPUT
-                        or block.block_type == BlockType.OUTPUT
-                        or block.block_type == BlockType.AGENT
-                        or block.block_type == BlockType.AI
+                        or block.block_type
+                        in [
+                            BlockType.INPUT,
+                            BlockType.OUTPUT,
+                            BlockType.AGENT,
+                        ]
                     )
                 ):
                     raise ValueError(

autogpt_platform/backend/backend/executor/manager.py

@@ -938,6 +938,11 @@ class ExecutionManager(AppService):
             else:
                 nodes_input.append((node.id, input_data))
 
+        if not nodes_input:
+            raise ValueError(
+                "No starting nodes found for the graph, make sure an AgentInput or blocks with no inbound links are present as starting nodes."
+            )
+
         graph_exec_id, node_execs = self.db_client.create_graph_execution(
             graph_id=graph_id,
             graph_version=graph.version,