feat: Model to track node states and has_ended flag (#94)

### TL;DR

Added an `ExecutionSummary` class to track execution state with comprehensive test coverage.

### What changed?

- Created a new `ExecutionSummary` dataclass in `api_server_sql.py` that tracks:
  - Total number of nodes
  - Number of ended nodes
  - Whether the entire execution has ended
- Added a `count_node_status` method to update counters based on node status
- Created a new test file `test_api_server_sql.py`.

### How to test?

Run the new test suite:
```
pytest tests/test_api_server_sql.py
```

### Why make this change?

This will be used in upstream PRs to calculate execution summary for runs and root nodes.

Author: yuechao-qin

cloud_pipelines_backend/api_server_sql.py

@@ -475,6 +475,22 @@ class ArtifactNodeIdResponse:
     id: bts.IdType
 
 
+@dataclasses.dataclass(kw_only=True)
+class ExecutionStatusSummary:
+    total_executions: int = 0
+    ended_executions: int = 0
+    has_ended: bool = False
+
+    def count_execution_status(
+        self, *, status: bts.ContainerExecutionStatus, count: int
+    ) -> None:
+        self.total_executions += count
+        if status in bts.CONTAINER_STATUSES_ENDED:
+            self.ended_executions += count
+
+        self.has_ended = self.ended_executions == self.total_executions
+
+
 @dataclasses.dataclass
 class GetGraphExecutionStateResponse:
     child_execution_status_stats: dict[bts.IdType, dict[str, int]]

tests/test_api_server_sql.py

@@ -0,0 +1,57 @@
+from cloud_pipelines_backend import backend_types_sql as bts
+from cloud_pipelines_backend.api_server_sql import ExecutionStatusSummary
+
+
+class TestExecutionStatusSummary:
+    def test_initial_state(self):
+        summary = ExecutionStatusSummary()
+        assert summary.total_executions == 0
+        assert summary.ended_executions == 0
+        assert summary.has_ended is False
+
+    def test_accumulate_all_ended_statuses(self):
+        """Add each ended status with 2^i count for robust uniqueness."""
+        summary = ExecutionStatusSummary()
+        ended_statuses = sorted(bts.CONTAINER_STATUSES_ENDED, key=lambda s: s.value)
+        expected_total = 0
+        expected_ended = 0
+        for i, status in enumerate(ended_statuses):
+            count = 2**i
+            summary.count_execution_status(status=status, count=count)
+            expected_total += count
+            expected_ended += count
+            assert summary.total_executions == expected_total
+            assert summary.ended_executions == expected_ended
+            assert summary.has_ended is True
+
+    def test_accumulate_all_in_progress_statuses(self):
+        """Add each in-progress status with 2^i count for robust uniqueness."""
+        summary = ExecutionStatusSummary()
+        in_progress_statuses = sorted(
+            set(bts.ContainerExecutionStatus) - bts.CONTAINER_STATUSES_ENDED,
+            key=lambda s: s.value,
+        )
+        expected_total = 0
+        for i, status in enumerate(in_progress_statuses):
+            count = 2**i
+            summary.count_execution_status(status=status, count=count)
+            expected_total += count
+            assert summary.total_executions == expected_total
+            assert summary.ended_executions == 0
+            assert summary.has_ended is False
+
+    def test_accumulate_all_statuses(self):
+        """Add every status with 2^i count. Summary math must be exact."""
+        summary = ExecutionStatusSummary()
+        all_statuses = sorted(bts.ContainerExecutionStatus, key=lambda s: s.value)
+        expected_total = 0
+        expected_ended = 0
+        for i, status in enumerate(all_statuses):
+            count = 2**i
+            expected_total += count
+            if status in bts.CONTAINER_STATUSES_ENDED:
+                expected_ended += count
+            summary.count_execution_status(status=status, count=count)
+            assert summary.total_executions == expected_total
+            assert summary.ended_executions == expected_ended
+            assert summary.has_ended == (expected_ended == expected_total)