fix(guardrails): satisfy ruff format and the test-tree lint

Two leftovers from narrowing this PR, both caught by CI's lint job:

- `_bin_pack_bedrock_content` kept its `lengths` tuple wrapped over three
  lines after the image dimension came out, but the expression fits on one
  again, so `ruff format --check` wanted it back. It now matches the base
  byte for byte and the function drops out of the diff entirely.
- the test module's top-level `from datetime import datetime, timezone`
  had no remaining user: the only `datetime` reference resolves through
  the local `import datetime` inside
  `test_load_credentials_assumes_role_with_external_id`, so the top-level
  form was shadowed and tripped F811.

litellm/proxy/guardrails/guardrail_hooks/bedrock_guardrails.py

@@ -1649,9 +1649,7 @@ class BedrockGuardrail(CustomGuardrail, BaseAWSLLM):
         if not content:
             return (tuple(content),)
 
-        lengths: Final = tuple(
-            len((item.get("text") or BedrockTextContent()).get("text") or "") for item in content
-        )
+        lengths: Final = tuple(len((item.get("text") or BedrockTextContent()).get("text") or "") for item in content)
 
         def assign(carried: tuple[int, int], length: int) -> tuple[int, int]:
             batch_index, used = carried

tests/test_litellm/proxy/guardrails/guardrail_hooks/test_bedrock_guardrails.py

@@ -6,7 +6,6 @@ import asyncio
 import base64
 import json
 import sys
-from datetime import datetime, timezone
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import httpx