fix: avoid panics on unsupported logger types

[happysnaker]

Summary

• stop treating unsupported Client.Logger values as a runtime panic
• fall back to no-op internal logging when the logger does not implement Logger or LeveledLogger
• add regression coverage showing requests and log hooks still complete without crashing

Why

The current Client.logger() path panics during request execution when Client.Logger is set to an unsupported type. That turns a configuration mistake into a production crash.

This patch keeps the existing supported logger behavior unchanged, but treats unsupported logger values the same way as no logger for internal logging paths. Requests still complete, hooks still run, and applications no longer crash at runtime because of an invalid logger assignment.

Closes #243.

[hashicorp-cla-app]

All committers have signed the CLA.

[happysnaker]

This should be ready for review on my side.

I have already signed the HashiCorp CLA previously, but the license/cla check is still showing as pending right after PR creation, so I'm waiting for that status to refresh.

Change summary:

• stop treating unsupported Client.Logger values as a runtime panic
• fall back to no-op internal logging for unsupported logger types
• add regression coverage showing requests and log hooks still complete without crashing