fix: fix Jupyter event loop conflict

[laurencap]

fix: avoid nested event loop in AwsQuantumTask.result() in Jupyter

Issue #, if available:

Fixes amazon-braket/amazon-braket-examples#857

Written with Kiro Opus 5.5.

Description of changes:

Bug: Calling AwsQuantumTask.result() from a thread with a running event loop (e.g. a Jupyter cell) nested run_until_complete() on that loop via nest_asyncio. On Python 3.12 this logs "RuntimeError: cannot enter context: ... is already entered" from Task.__step, even though the result is returned.

New: When a loop is already running, result() now polls on a worker thread with its own event loop via asyncio.run() instead of nesting. Interrupting the kernel (e.g. from jupyter) will cancel the polling loop.
_get_future() creates the polling task with loop.create_task(), which removes the need for _create_future() and fixes the issue in the case where the customer calls await task.async_result().

Follow up work: we no longer need nest_asyncio.

Testing done:

I was able to reliably reproduce the error by running this example notebook: https://github.com/amazon-braket/amazon-braket-examples/blob/main/examples/braket_features/Allocating_Qubits_on_QPU_Devices.ipynb

With these changes, the issue no longer appears.

Added new unit tests.

Merge Checklist

General

• I have read the CONTRIBUTING doc
• I used the PR title format described in CONTRIBUTING
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (428fefa) to head (db447d9).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1357   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          171       171           
  Lines        11648     11667   +19     
  Branches      1538      1539    +1     
=========================================
+ Hits         11648     11667   +19     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.