fix(guardrails): reject invalid UK NINO prefix letters

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/proxy/guardrails/guardrail_hooks/litellm_content_filter/patterns.json

@@ -628,7 +628,7 @@
     {
       "name": "uk_nino",
       "display_name": "National Insurance Number (UK)",
-      "pattern": "\\b(?!BG|GB|KN|NK|NT|TN|ZZ)[A-CEGHJ-PR-TW-Z]{2}\\s?\\d{2}(?:\\s?\\d{2}){2}\\s?[A-D]\\b",
+      "pattern": "\\b(?!BG|GB|KN|NK|NT|TN|ZZ)[ABCEGHJKLMNOPRSTWXYZ][ABCEGHJKLMNPRSTWXYZ]\\s?\\d{2}(?:\\s?\\d{2}){2}\\s?[A-D]\\b",
       "category": "UK PII Patterns",
       "description": "Detects UK National Insurance numbers with valid prefixes and suffixes",
       "keyword_pattern": "\\b(?:national\\s*insurance\\s*number|national\\s*insurance|NI\\s*number|NI\\s*no\\.?|NINO)\\b",

tests/unit/proxy/guardrails/guardrail_hooks/content_filter/test_uk_patterns.py

@@ -107,6 +107,37 @@ async def test_invalid_uk_pii_format_passes_unchanged(
     assert valid_value not in output
 
 
+@pytest.mark.parametrize(
+    "invalid_prefix",
+    (
+        "DA",
+        "FA",
+        "IA",
+        "QA",
+        "UA",
+        "VA",
+        "AD",
+        "AF",
+        "AI",
+        "AO",
+        "AQ",
+        "AU",
+        "AV",
+    ),
+)
+@pytest.mark.asyncio
+async def test_uk_nino_rejects_disallowed_prefix_letters(invalid_prefix: str) -> None:
+    guardrail: Final = _guardrail_for("uk_nino")
+    text: Final = f"national insurance number: {invalid_prefix}123456A"
+    result: Final = await guardrail.apply_guardrail(
+        inputs={"texts": [text]},
+        request_data={},
+        input_type="request",
+    )
+
+    assert result["texts"][0] == text
+
+
 def test_uk_policy_templates_reference_available_prebuilt_patterns() -> None:
     patterns_file: Final = REPO_ROOT / "litellm/proxy/guardrails/guardrail_hooks/litellm_content_filter/patterns.json"
     pattern_names: Final = {