fix: set comprehend_client when ComprehendFilterAgent creates a default client

[YuemengZheng]

Issue Link (REQUIRED)

Fixes #359

Summary

Changes

When no client is passed to ComprehendFilterAgent, the constructor created the boto3 Comprehend client but assigned it to self.client instead of self.comprehend_client. Since detect_sentiment / detect_pii_entities / detect_toxic_content all use self.comprehend_client, calling process_request raised:
'ComprehendFilterAgent' object has no attribute 'comprehend_client'.

Assign the default-created client to self.comprehend_client so the agent works without an explicitly provided client. Added a regression test that patches boto3.client and asserts comprehend_client is set when client=None.

User experience

Before: constructing a ComprehendFilterAgent without a client (the documented default) crashed on the first request. After: the agent creates its own Comprehend client and runs normally.

Checklist

• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• I have linked this PR to an existing issue (required)