Python: improve .env handling and observability samples (#4032)

* Python: improve .env precedence and observability samples

- Switch load_settings to explicit precedence: overrides -> explicit .env -> environment -> defaults\n- Raise when env_file_path is provided but missing\n- Update settings docs and tests for new behavior\n- Refresh observability samples and README guidance for env loading options\n\nCloses #3864\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fixed some imports

* Fix load_settings CI regressions

Allow explicit env_file_path values that exist but are not regular files (for example /dev/null) by checking path existence before dotenv parsing, and restore a dict accumulator with typed return cast to satisfy mypy.

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

* Avoid implicit dotenv in observability

Only load dotenv in observability helpers when env_file_path is explicitly provided, and remove test os.devnull workarounds that are no longer necessary.

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

---------

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

python/packages/core/tests/azure/test_azure_assistants_client.py

@@ -130,7 +130,7 @@ def test_azure_assistants_client_init_missing_deployment_name(azure_openai_unit_
     """Test AzureOpenAIAssistantsClient initialization with missing deployment name."""
     with pytest.raises(ServiceInitializationError):
         AzureOpenAIAssistantsClient(
-            api_key=azure_openai_unit_test_env.get("AZURE_OPENAI_API_KEY", "test-key"), env_file_path="nonexistent.env"
+            api_key=azure_openai_unit_test_env.get("AZURE_OPENAI_API_KEY", "test-key")
         )
 
 

python/packages/core/tests/azure/test_azure_chat_client.py

@@ -95,7 +95,6 @@ def test_init_endpoint(azure_openai_unit_test_env: dict[str, str]) -> None:
 def test_init_with_empty_deployment_name(azure_openai_unit_test_env: dict[str, str]) -> None:
     with pytest.raises(ServiceInitializationError):
         AzureOpenAIChatClient(
-            env_file_path="test.env",
         )
 
 
@@ -103,7 +102,6 @@ def test_init_with_empty_deployment_name(azure_openai_unit_test_env: dict[str, s
 def test_init_with_empty_endpoint_and_base_url(azure_openai_unit_test_env: dict[str, str]) -> None:
     with pytest.raises(ServiceInitializationError):
         AzureOpenAIChatClient(
-            env_file_path="test.env",
         )
 
 
@@ -126,7 +124,6 @@ def test_serialize(azure_openai_unit_test_env: dict[str, str]) -> None:
         "api_key": azure_openai_unit_test_env["AZURE_OPENAI_API_KEY"],
         "api_version": azure_openai_unit_test_env["AZURE_OPENAI_API_VERSION"],
         "default_headers": default_headers,
-        "env_file_path": "test.env",
     }
 
     azure_chat_client = AzureOpenAIChatClient.from_dict(settings)

python/packages/core/tests/azure/test_azure_responses_client.py

@@ -112,7 +112,6 @@ def test_init_with_default_header(azure_openai_unit_test_env: dict[str, str]) ->
 def test_init_with_empty_model_id(azure_openai_unit_test_env: dict[str, str]) -> None:
     with pytest.raises(ServiceInitializationError):
         AzureOpenAIResponsesClient(
-            env_file_path="test.env",
         )