fix: make TrackingContext thread-safe

[alyabouzaid]

Issue #, if available: Fixes #1319

Description of changes:

TrackingContext shares one process-global set of trackers; broadcast_event() (called on every get_quantum_task()) iterates it while Tracker context managers on other threads add/remove entries, which intermittently raises RuntimeError: Set changed size during iteration. This change guards the set with a threading.Lock and has broadcast_event iterate over a snapshot, so the lock is never held across tracker callbacks.

Testing done:

• New unit test where a tracker deregisters itself during a broadcast; it fails on main with the RuntimeError above and passes with this change.
• Ran the threaded reproduction script from TrackingContext is not thread-safe: broadcast_event iterates _trackers without a lock → RuntimeError: Set changed size during iteration #1319 (1,000,000 broadcasts under concurrent Tracker enter/exit churn): no error after the fix.
• pytest test/unit_tests/braket/tracking test/unit_tests/braket/aws/test_aws_session.py: 110 passed, 4 xfailed. ruff format --check and ruff check src clean.

AI disclosure: this PR was prepared with the assistance of Claude Code (Anthropic's Claude); I review and test all changes.

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

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 (df95926) to head (bf5ec8a).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1321   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          171       171           
  Lines        11550     11556    +6     
  Branches      1511      1511           
=========================================
+ Hits         11550     11556    +6     

☔ 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.