Python: updated integration tests and guidance (#4181)

* updated integration tests and guidance

* fixed merge test

* updated integration tests

* fix: remove duplicate --dist loadfile flag from pytest-xdist config

Only one --dist mode can be active at a time; the second value silently
overrides the first. Keep --dist worksteal (dynamic load balancing) and
remove the redundant --dist loadfile from all workflow files and
pyproject.toml configs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* docs: add keep-in-sync notes for merge and integration test workflows

Both python-merge-tests.yml and python-integration-tests.yml share the
same parallel job structure. Added sync reminders in workflow file
comments, the python-testing SKILL.md, and CODING_STANDARD.md.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* refactor: remove RUN_INTEGRATION_TESTS flag

Integration test gating now uses two mechanisms:
- `@pytest.mark.integration` for test selection via `-m` filtering
- `skip_if_*_disabled` for credential/service availability checks

The RUN_INTEGRATION_TESTS env var was redundant since the marker handles
selection and the skip decorators already check for actual credentials.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: sync missing env vars from merge-tests to integration-tests

Add OPENAI_EMBEDDINGS_MODEL_ID and AZURE_OPENAI_EMBEDDING_DEPLOYMENT_NAME
to python-integration-tests.yml to match python-merge-tests.yml.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: remove remaining RUN_INTEGRATION_TESTS from embedding tests and docs

Missed test_openai_embedding_client.py and vector-stores README in the
earlier cleanup.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* set functions tests to 3.10

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/packages/durabletask/tests/integration_tests/conftest.py

@@ -289,9 +289,6 @@ def pytest_configure(config: pytest.Config) -> None:
 
 def pytest_collection_modifyitems(config: pytest.Config, items: list[pytest.Item]) -> None:
     """Skip tests based on markers and environment availability."""
-    run_integration = os.getenv("RUN_INTEGRATION_TESTS", "false").lower() == "true"
-    skip_integration = pytest.mark.skip(reason="RUN_INTEGRATION_TESTS not set to 'true'")
-
     # Check Azure OpenAI environment variables
     azure_openai_vars = ["AZURE_OPENAI_ENDPOINT", "AZURE_OPENAI_CHAT_DEPLOYMENT_NAME"]
     azure_openai_available = all(os.getenv(var) for var in azure_openai_vars)
@@ -308,8 +305,6 @@ def pytest_collection_modifyitems(config: pytest.Config, items: list[pytest.Item
     skip_redis = pytest.mark.skip(reason="Redis is not available at redis://localhost:6379")
 
     for item in items:
-        if "integration_test" in item.keywords and not run_integration:
-            item.add_marker(skip_integration)
         if "requires_azure_openai" in item.keywords and not azure_openai_available:
             item.add_marker(skip_azure_openai)
         if "requires_dts" in item.keywords and not dts_available: