PYTHON-6030 Consolidate _OIDCAuthenticator classes into shared base class

[NoahStapp]

PYTHON-6030

Changes in this PR

Consolidate existing sync and async _OIDCAuthenticator non-IO code into a shared base class.

Test Plan

Existing test suite.

Checklist

Checklist for Author

• [ ] Did you update the changelog (if necessary)?
• Is there test coverage?
• [ ] Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[Copilot]

🟢 Approval recommended

Static review supports a behavior-preserving refactor with no blocking issues; tests were not run.

No open issues 🎉

What changed in this PR

Consolidates shared, non-I/O OIDC authentication logic into a base class while keeping I/O in the synchronous and asynchronous implementations.

Changes:

• Moves common state and command-building helpers into _OIDCAuthenticatorBase.
• Updates subclasses to inherit shared behavior and pass token generation IDs to invalidation.

File	Description
pymongo/​synchronous/​auth_oidc.py	Uses the shared base in the generated synchronous implementation.
pymongo/​auth_oidc_shared.py	Defines the shared authenticator state and non-I/O helpers.
pymongo/​asynchronous/​auth_oidc.py	Uses the shared base while retaining asynchronous I/O.

_{Review effort: Balanced}