fix: Return response in bedrock guardrail async_post_call_success_hook

[orolega]

Title

Improve Bedrock Guardrails: Async Hook Fix

Relevant issues

N/A

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• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
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Type

🐛 Bug Fix
✅ Test

Changes

Previously, the async_post_call_success_hook method in BedrockGuardrail was modifying the response in-place but not returning it. Now the response is being lost when using a post_call Bedrock guardrail as it is no longer being modified in place. This results in no response being sent back to the client.

This fix ensures the response object is properly returned after applying guardrail checks and masking, maintaining consistency with other guardrail implementations (AIM, Pangea, etc.) and the pre_call hook behavior.

• Add return statement in async_post_call_success_hook
• Add test to verify response is returned correctly
• Update test comment for clarity

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