🐛 fix(aio): avoid RuntimeError constructing AuthByWebBrowser on Py 3.14

[13steinj]

The old "cache asyncio.get_event_loop() as a member, reuse it" idiom is incorrect (between time-of-cache and time-of-use the active event loop can change). As a result of this behavior Python deprecated asyncio.get_event_loop() and the event-loop policy system, and started raising a RuntimeError in Python 3.14 when there is no running (or previously-set) event loop, instead of silently creating one.

Previously the call was unconditional in a constructor, so constructing outside of a coroutine/event loop now crashes (e.g. in a test).

Drop the cached self._event_loop entirely and call asyncio.get_running_loop() inline at the relevant call-sites that are all only reachable from coroutines anyway.

Update every mock within test/unit/aio/test_auth_webbrowser_async.py to patch the right method call.

Add python 3.14 to the test-aio CI matrix (job is currently disabled as of 7d1356c / #2718, so this has no effect until it's re-enabled).

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-3890777: AuthByWebBrowser incorrectly caches asyncio event loop #2988

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • I don't know if this counts or not because some of these test are disabled in CI but some downstream consumers end up running your tests anyway.
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am modifying authorization mechanisms
   • I am adding new credentials
   • I am modifying OCSP code
   • I am adding a new dependency

3. Please describe how your code solves the related issue.

   Uses the new idiom for acting on the running event loop.

4. (Optional) PR for stored-proc connector:
   N/A

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[13steinj]

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[13steinj]

recheck