feat(bx): invoice ledger balance service with unit tests

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

backend/tests/test_bx_invoice_ledger.py

@@ -0,0 +1,41 @@
+"""发票额度流水汇总与退回幂等。"""
+from decimal import Decimal
+
+from app.services.bx.invoice_ledger import (
+    ENTRY_CREDIT_RELEASE,
+    ENTRY_CREDIT_UPLOAD,
+    ENTRY_DEBIT_DELETE,
+    ENTRY_DEBIT_REIMBURSE,
+    should_skip_release,
+    summarize_entries,
+)
+
+
+def test_summarize_upload_and_reimburse():
+    rows = [
+        {"entry_type": ENTRY_CREDIT_UPLOAD, "amount": Decimal("1000")},
+        {"entry_type": ENTRY_DEBIT_REIMBURSE, "amount": Decimal("300")},
+    ]
+    s = summarize_entries(rows)
+    assert s["total_amount"] == Decimal("1000")
+    assert s["used_amount"] == Decimal("300")
+    assert s["balance"] == Decimal("700")
+
+
+def test_summarize_delete_and_release():
+    rows = [
+        {"entry_type": ENTRY_CREDIT_UPLOAD, "amount": Decimal("500")},
+        {"entry_type": ENTRY_DEBIT_DELETE, "amount": Decimal("100")},
+        {"entry_type": ENTRY_DEBIT_REIMBURSE, "amount": Decimal("200")},
+        {"entry_type": ENTRY_CREDIT_RELEASE, "amount": Decimal("200")},
+    ]
+    s = summarize_entries(rows)
+    assert s["total_amount"] == Decimal("400")
+    assert s["used_amount"] == Decimal("0")
+    assert s["balance"] == Decimal("400")
+
+
+def test_release_idempotent_guard():
+    assert should_skip_release(has_debit=True, has_release=True) is True
+    assert should_skip_release(has_debit=True, has_release=False) is False
+    assert should_skip_release(has_debit=False, has_release=False) is True