Fix HTTP proxy Basic Auth credential decoding

[sol-hermes85]

Summary

Fixes Basic Auth credential decoding in the HTTP proxy module.

base64.b64decode() returns bytes under Python 3, but the existing code splits with a string delimiter. That raises an exception, is swallowed, and causes valid Basic Auth credentials to be logged as Invalid auth-token submitted.

This PR:

• decodes the Basic Auth token before splitting
• splits on the first colon only, preserving passwords that contain :
• updates the HTTP proxy test to assert the captured username/password values

Testing

• uvx pre-commit run --all-files
• pytest opencanary/test/test_httpproxy.py -q

For the pytest run I started a minimal local OpenCanary instance with only the HTTP proxy enabled and logging to /var/tmp/opencanary.log, matching the integration test expectation.