Fix ApiClient finalizer during interpreter shutdown

[abhinavrastogi2627]

Summary

• prevent ApiClient.__del__ from raising while Python clears client state during interpreter shutdown
• add coverage for a finalizer invoked after cleanup attributes have been removed

Closes #719

Validation

• pytest -q tests/test_InfluxDBClientAuthorization.py
• flake8 --isolated --max-line-length=120 --select=E501 influxdb_client/_sync/api_client.py tests/test_InfluxDBClientAuthorization.py
• git diff --check

The full pytest -q command was attempted after installing the test, extra, and async dependencies. It requires an InfluxDB server at localhost:8086; collection proceeded, then the integration suite failed at its first server connection because no local server is available.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (7ed4ecf) to head (4bc8d34).
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #720      +/-   ##
==========================================
- Coverage   90.46%   90.41%   -0.05%     
==========================================
  Files          40       40              
  Lines        3555     3568      +13     
==========================================
+ Hits         3216     3226      +10     
- Misses        339      342       +3     

☔ 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.

[abhinavrastogi2627]

Addressed the coverage advisory in 4bc8d34 with regression tests for signout failure, thread-pool close/join failures, and HTTP-pool cleanup failure. The tests verify later cleanup still runs and no teardown exception escapes. All executable lines in the changed finalizer (lines 86-106) are covered by the local coverage run.

Validation: python -m coverage run --include='*/influxdb_client/_sync/api_client.py' -m unittest tests.test_InfluxDBClientAuthorization passed all 5 tests; changed-test-file E501 checks and git diff --check passed. No production code changed in this follow-up. The full integration suite still requires the InfluxDB service noted in the PR; I have not claimed a local integration pass or installed the upstream Codecov app.