fix: OpenCode instances fail with 'no port configured' error

For terminal-only tools like OpenCode, default_port is 0 which is falsy
in Python. The code incorrectly treated port 0 as 'not configured' and
marked the instance as error. Now we only check if tool_type exists,
and default to port 0. Terminal tools skip tunnel creation anyway.

apps/api/src/api/tool_instances.py

@@ -736,17 +736,16 @@ async def start_instance(
 
     # Get tool type for default port
     tool_type = await session.get(ToolType, instance.tool_type_id)
-    if not tool_type or not tool_type.default_port:
-        logger.error("Tool type %s has no default_port configured. Cannot create tunnel.", 
-                    instance.tool_type_id)
+    if not tool_type:
+        logger.error("Tool type %s not found", instance.tool_type_id)
         instance.status = "error"
         await session.commit()
         return {
             "status": "error",
-            "error": f"Tool type '{tool_type.name if tool_type else 'unknown'}' has no port configured",
+            "error": f"Tool type '{instance.tool_type_id}' not found",
         }
     
-    instance_port = tool_type.default_port
+    instance_port = tool_type.default_port or 0
     logger.info("Tool type for instance %s: name=%s, default_port=%s, interface_type=%s", 
                 instance.id, tool_type.name, instance_port, tool_type.interface_type)