add notification POST response logging

[sleibach]

The logged info is needed when support tickets need to be opened regarding business notifications.

[StoilAtSAP]

Tested this PR and looks good to merge.

Testing done:

• All 55 plugin tests pass ✅
• Integration with incidents-app works with no regressions ✅
• Code changes are minimal and well-structured

What it adds:

• Captures the full HTTP response from ANS API (was previously discarded)
• Logs response body + headers at debug level for troubleshooting
• Returns the response object so applications can use it if needed
• Adds Accept: application/json header

Risk: Very low - backward compatible, only affects debug logging and return value when outbox is disabled.

This solves a real support problem when debugging notification issues. 👍