agent_aws: handle bytes values in special agent JSON serializer

[terashiman]

Summary

The AWS special agent's custom JSON serializer (datetime_serializer in agent_aws.py) only handles datetime objects and raises TypeError for anything else. AWS WAFv2's ByteMatchStatement.SearchString field is decoded by boto3 as Python bytes (e.g. b'/admin/' for a rule blocking that URI path), which is a completely ordinary WAFv2 rule configuration. When such a rule exists on a monitored Web ACL, serializing the wafv2_web_acl section raises an unhandled TypeError, and the special agent exits with code 1 before finishing output. This causes Check_MK Discovery to report the WAFv2 (and any other not-yet-emitted) services as vanished/critical, even though AWS connectivity and credentials are fine.

Reproduced with --debug:

TypeError: b'/admin/' is not JSON serializable

Fix

Add a bytes branch to datetime_serializer that decodes to str (UTF-8, errors="replace") instead of falling through to the TypeError.

Test plan

• Ran agent_aws manually against a WAFv2 Web ACL containing a ByteMatchStatement rule; reproduced TypeError: b'...' is not JSON serializable and exit code 1 on the unpatched agent
• Applied the fix; reran the same invocation, got exit code 0 and a correctly decoded SearchString value in the wafv2_web_acl section
• Re-enabled WAFv2 monitoring on the affected host and confirmed Check_MK Discovery and all AWS/WAFv2 services report OK

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[terashiman]

I have read the CLA Document and I hereby sign the CLA.