Fix: Only include updatedInput when explicitly provided in permission callbacks

Fixes #1063

When a permission callback returns `PermissionResultAllow()` without
the `updated_input` parameter, the SDK should not include `updatedInput`
in the control_response. Previously, the SDK always included this field,
either with the modified input or falling back to the original input,
which caused MCP tools to receive empty {} arguments instead of Claude's
original arguments.

Changes:
- Modified query.py to only include updatedInput when explicitly set
- Added regression test to verify updatedInput is omitted when not provided
- Existing test for input modification still passes

Before this fix:
```json
{
  "behavior": "allow",
  "updatedInput": {}  // Bug: Always included even when not set
}
```

After this fix:
```json
{
  "behavior": "allow"  // updatedInput only included when explicitly provided
}
```

src/claude_agent_sdk/_internal/query.py

@@ -233,14 +233,10 @@ class Query:
 
                 # Convert PermissionResult to expected dict format
                 if isinstance(response, PermissionResultAllow):
-                    response_data = {
-                        "behavior": "allow",
-                        "updatedInput": (
-                            response.updated_input
-                            if response.updated_input is not None
-                            else original_input
-                        ),
-                    }
+                    response_data = {"behavior": "allow"}
+                    # Only include updatedInput if explicitly provided by the user
+                    if response.updated_input is not None:
+                        response_data["updatedInput"] = response.updated_input
                     if response.updated_permissions is not None:
                         response_data["updatedPermissions"] = [
                             permission.to_dict()

tests/test_tool_callbacks.py

@@ -96,6 +96,13 @@ class TestToolPermissionCallbacks:
         response = transport.written_messages[0]
         assert '"behavior": "allow"' in response
 
+        # IMPORTANT: Verify updatedInput is NOT included when not provided
+        # This is the regression test for issue #1063
+        assert '"updatedInput"' not in response, (
+            "updatedInput should not be included when PermissionResultAllow() "
+            "is returned without updated_input parameter"
+        )
+
     @pytest.mark.asyncio
     async def test_permission_callback_deny(self):
         """Test callback that denies tool execution."""