Fix integration test worker crashes on Python 3.13

Three changes to prevent pytest-xdist workers from crashing during
Azure Functions integration tests:

1. Add `start_new_session=True` to subprocess on Linux so signals
   (e.g. from test-timeout) cannot propagate between the func host
   and the xdist worker process.

2. Add an overall 100-second budget to the fixture setup loop so
   the retry logic never exceeds the 120-second test timeout. When
   pytest-timeout's thread method fires during fixture setup and the
   thread doesn't respond, it calls os._exit() which kills the
   xdist worker – this is the root cause of the "Not properly
   terminated" crashes.

3. Remove the `UV_PYTHON: "3.10"` workaround from both workflow
   files so integration tests actually run on Python 3.13.

Co-authored-by: larohra <41490930+larohra@users.noreply.github.com>

.github/workflows/python-integration-tests.yml

@@ -170,7 +170,6 @@ jobs:
     environment: integration
     timeout-minutes: 60
     env:
-      UV_PYTHON: "3.10"
       OPENAI_CHAT_MODEL_ID: ${{ vars.OPENAI__CHATMODELID }}
       OPENAI_RESPONSES_MODEL_ID: ${{ vars.OPENAI__RESPONSESMODELID }}
       OPENAI_API_KEY: ${{ secrets.OPENAI__APIKEY }}