fix: replace deprecated utcfromtimestamp in google-auth-oauthlib

[chalmerlowe]

This PR replaces the usage of the deprecated datetime.datetime.utcfromtimestamp in the google-auth-oauthlib package and testdata.

Using datetime.datetime.fromtimestamp(session.token["expires_at"], datetime.timezone.utc).replace(tzinfo=None) avoids the deprecation warning in Python 3.12 while preserving backwards compatibility by returning a naive datetime object.

Fixes #15433

---

PR created automatically by Jules for task 232508796818525143 started by @chalmerlowe

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[gemini-code-assist]

Code Review

This pull request updates the credentials_from_session function in helpers.py across the codebase to replace the deprecated datetime.datetime.utcfromtimestamp method. The new implementation uses datetime.datetime.fromtimestamp with datetime.timezone.utc and strips the timezone information to maintain the existing behavior of using naive datetime objects. I have no feedback to provide as there were no review comments.