fix: use self.comprehend_client consistently in ComprehendFilterAgent

[themavik]

Summary

ComprehendFilterAgent raises AttributeError: 'ComprehendFilterAgent' object has no attribute 'comprehend_client' when instantiated without providing a custom client in options.

Root Cause

In __init__, when options.client is None, the boto3 Comprehend client is assigned to self.client instead of self.comprehend_client:

# Before (broken)
if options.client:
    self.comprehend_client = options.client  # correct
else:
    if options.region:
        self.client = boto3.client(...)  # BUG: wrong attribute name
    else:
        self.client = boto3.client(...)  # BUG: wrong attribute name

All downstream methods (detect_sentiment, detect_pii_entities, detect_toxic_content) reference self.comprehend_client, so any agent created without a pre-built client immediately fails.

Fix

Changed both else branches to assign to self.comprehend_client:

if options.client:
    self.comprehend_client = options.client
else:
    if options.region:
        self.comprehend_client = boto3.client(
            'comprehend',
            region_name=options.region or os.environ.get('AWS_REGION')
        )
    else:
        self.comprehend_client = boto3.client('comprehend')

Test Plan

• Instantiate ComprehendFilterAgent with client=None and a region — no longer raises AttributeError
• Instantiate with client=None and no region — uses default boto3 region, no error
• Instantiate with a custom client — behavior unchanged

Fixes #359