[Core] Make _enforce_https a module level function

[pvaneck]

Closes: #45402

[Copilot]

Pull request overview

Refactors bearer token credential policies to make _enforce_https a module-level helper, avoiding cross-class protected member access in async policies and aligning sync/async implementations.

Changes:

• Introduced module-level _enforce_https(request) helper in both azure-core and corehttp authentication policy modules.
• Updated sync and async bearer token policies to call _enforce_https directly instead of a base-class @staticmethod.
• Simplified async policy imports by removing the dependency on _BearerTokenCredentialPolicyBase solely for HTTPS enforcement.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
sdk/core/corehttp/corehttp/runtime/policies/_authentication_async.py	Switches async policy HTTPS enforcement to module-level _enforce_https.
sdk/core/corehttp/corehttp/runtime/policies/_authentication.py	Moves _enforce_https out of _BearerTokenCredentialPolicyBase into module scope and updates call site.
sdk/core/azure-core/azure/core/pipeline/policies/_authentication_async.py	Replaces protected cross-class call with direct _enforce_https usage.
sdk/core/azure-core/azure/core/pipeline/policies/_authentication.py	Adds module-level _enforce_https, removes the old staticmethod, and updates call site.