Enhanced GraphEngine Pause Handling (#28196)

This commit: 

1. Convert `pause_reason` to `pause_reasons` in `GraphExecution` and relevant classes. Change the field from a scalar value to a list that can contain multiple `PauseReason` objects, ensuring all pause events are properly captured.
2. Introduce a new `WorkflowPauseReason` model to record reasons associated with a specific `WorkflowPause`.

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: -LAN- <laipz8200@outlook.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

api/core/workflow/runtime/graph_runtime_state.py

@@ -10,6 +10,7 @@ from typing import Any, Protocol
 from pydantic.json import pydantic_encoder
 
 from core.model_runtime.entities.llm_entities import LLMUsage
+from core.workflow.entities.pause_reason import PauseReason
 from core.workflow.runtime.variable_pool import VariablePool
 
 
@@ -46,7 +47,11 @@ class ReadyQueueProtocol(Protocol):
 
 
 class GraphExecutionProtocol(Protocol):
-    """Structural interface for graph execution aggregate."""
+    """Structural interface for graph execution aggregate.
+
+    Defines the minimal set of attributes and methods required from a GraphExecution entity
+    for runtime orchestration and state management.
+    """
 
     workflow_id: str
     started: bool
@@ -54,6 +59,7 @@ class GraphExecutionProtocol(Protocol):
     aborted: bool
     error: Exception | None
     exceptions_count: int
+    pause_reasons: list[PauseReason]
 
     def start(self) -> None:
         """Transition execution into the running state."""