feat(backend): add draft save helpers for plan follow status drafting

Introduce min-field validation and follow_status resolution for saveMode draft/formal.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_pl_plan_draft_save.py

@@ -0,0 +1,58 @@
+import pytest
+from fastapi import HTTPException
+
+from app.services.pl_plan_service import (
+    DRAFTABLE_FOLLOW_STATUSES,
+    assert_can_save_as_draft,
+    resolve_follow_status_on_save,
+    validate_draft_min_fields,
+)
+
+
+def test_validate_draft_min_fields_requires_belongs_company():
+    with pytest.raises(HTTPException) as exc:
+        validate_draft_min_fields(belongs_company="", create_date=None)
+    assert exc.value.status_code == 422
+    assert "归属公司" in str(exc.value.detail)
+
+
+def test_validate_draft_min_fields_allows_missing_create_date():
+    validate_draft_min_fields(belongs_company="shanxia", create_date=None)
+
+
+def test_resolve_follow_status_draft_always_drafting():
+    assert (
+        resolve_follow_status_on_save(
+            save_mode="draft", current="pending_arrange", is_create=False
+        )
+        == "drafting"
+    )
+    assert resolve_follow_status_on_save(save_mode="draft", current=None, is_create=True) == "drafting"
+
+
+def test_resolve_follow_status_formal_promotes_drafting():
+    assert (
+        resolve_follow_status_on_save(save_mode="formal", current=None, is_create=True)
+        == "pending_arrange"
+    )
+    assert (
+        resolve_follow_status_on_save(save_mode="formal", current="drafting", is_create=False)
+        == "pending_arrange"
+    )
+    assert (
+        resolve_follow_status_on_save(save_mode="formal", current="arranged", is_create=False)
+        == "arranged"
+    )
+    assert (
+        resolve_follow_status_on_save(save_mode="formal", current="wait_notice", is_create=False)
+        == "wait_notice"
+    )
+
+
+def test_assert_can_save_as_draft_rejects_arranged():
+    assert "drafting" in DRAFTABLE_FOLLOW_STATUSES
+    assert "pending_arrange" in DRAFTABLE_FOLLOW_STATUSES
+    with pytest.raises(HTTPException) as exc:
+        assert_can_save_as_draft("arranged")
+    assert exc.value.status_code == 400
+    assert "不可暂存" in str(exc.value.detail)