revert: point the samples at us-central1 again

[jhuleatt]

Why

#1327 changed example URLs, client code, and the Python task queue defaults in 25 samples from us-central1 to us-east4, on the assumption that the default region for Cloud Functions for Firebase had moved. It hasn't. In the Firebase CLI:

• HTTP, callable, scheduled, and task queue functions that don't set a region still deploy to us-central1 (FALLBACK_DEPLOYMENT_REGION in src/deploy/functions/services/index.ts).
• Since CLI v15.17.0 (firebase-tools#10414), new event-triggered functions default to the location of their event source. A Firestore database in nam5 gives us-central1, for example, and global sources such as Pub/Sub give us-east1.
• us-east4 is the new default for App Hosting and Data Connect, not for Functions.

None of the samples touched by #1327 set a region, so they deploy to us-central1. Their URLs, client code, and queue paths pointed at functions that don't exist. This PR puts the strings back.

What's kept from #1327

• The .value on SupportedRegion in the Python task queue samples, so queue_path and get_function_url get a string rather than an enum member.
• The requiresRole and requiresAPI import from the firebase-functions package root.
• The us-east1 example URLs in Node/quickstarts/https-time-server, which deploys to us-west1 and us-east1.

Notes

• Supersedes fix: deploy the samples to us-east4, where their URLs now point #1346, which took the other route and pinned every sample to us-east4. I'll close that one.
• The diff is 25 files and 39 lines, all us-east4 → us-central1. I checked with git diff origin/main that nothing else changed.
• fix(auth samples): keep the OAuth state in the __session cookie #1338 replaces the two affected lines in Node-1st-gen/spotify-auth/public/popup.html with relative URLs. Whichever of the two lands second needs a one-line rebase.
• String changes only; not deployed.

[gemini-code-assist]

Code Review

This pull request systematically updates the default region for Firebase Cloud Functions and emulator endpoints from us-east4 to us-central1 across multiple project examples, configuration files, setup scripts, and README documentation in both Node.js and Python. I have no feedback to provide as there are no review comments.