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/migrations/versions/2025_11_18_1859-7bb281b7a422_add_workflow_pause_reasons_table.py

@@ -0,0 +1,41 @@
+"""Add workflow_pauses_reasons table
+
+Revision ID: 7bb281b7a422
+Revises: 09cfdda155d1
+Create Date: 2025-11-18 18:59:26.999572
+
+"""
+
+from alembic import op
+import models as models
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "7bb281b7a422"
+down_revision = "09cfdda155d1"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        "workflow_pause_reasons",
+        sa.Column("id", models.types.StringUUID(), nullable=False),
+        sa.Column("created_at", sa.DateTime(), server_default=sa.text("CURRENT_TIMESTAMP"), nullable=False),
+        sa.Column("updated_at", sa.DateTime(), server_default=sa.text("CURRENT_TIMESTAMP"), nullable=False),
+
+        sa.Column("pause_id", models.types.StringUUID(), nullable=False),
+        sa.Column("type_", sa.String(20), nullable=False),
+        sa.Column("form_id", sa.String(length=36), nullable=False),
+        sa.Column("node_id", sa.String(length=255), nullable=False),
+        sa.Column("message", sa.String(length=255), nullable=False),
+
+        sa.PrimaryKeyConstraint("id", name=op.f("workflow_pause_reasons_pkey")),
+    )
+    with op.batch_alter_table("workflow_pause_reasons", schema=None) as batch_op:
+        batch_op.create_index(batch_op.f("workflow_pause_reasons_pause_id_idx"), ["pause_id"], unique=False)
+
+
+def downgrade():
+    op.drop_table("workflow_pause_reasons")