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/openai/test_openai_assistants_client.py

@@ -155,7 +155,7 @@ def test_init_missing_model_id(openai_unit_test_env: dict[str, str]) -> None:
     """Test OpenAIAssistantsClient initialization with missing model ID."""
     with pytest.raises(ServiceInitializationError):
         OpenAIAssistantsClient(
-            api_key=openai_unit_test_env.get("OPENAI_API_KEY", "test-key"), env_file_path="nonexistent.env"
+            api_key=openai_unit_test_env.get("OPENAI_API_KEY", "test-key")
         )
 
 
@@ -163,7 +163,7 @@ def test_init_missing_model_id(openai_unit_test_env: dict[str, str]) -> None:
 def test_init_missing_api_key(openai_unit_test_env: dict[str, str]) -> None:
     """Test OpenAIAssistantsClient initialization with missing API key."""
     with pytest.raises(ServiceInitializationError):
-        OpenAIAssistantsClient(model_id="gpt-4", env_file_path="nonexistent.env")
+        OpenAIAssistantsClient(model_id="gpt-4")
 
 
 def test_init_with_default_headers(openai_unit_test_env: dict[str, str]) -> None:

python/packages/core/tests/openai/test_openai_chat_client.py

@@ -98,7 +98,6 @@ def test_init_base_url_from_settings_env() -> None:
 def test_init_with_empty_model_id(openai_unit_test_env: dict[str, str]) -> None:
     with pytest.raises(ServiceInitializationError):
         OpenAIChatClient(
-            env_file_path="test.env",
         )
 
 
@@ -109,7 +108,6 @@ def test_init_with_empty_api_key(openai_unit_test_env: dict[str, str]) -> None:
     with pytest.raises(ServiceInitializationError):
         OpenAIChatClient(
             model_id=model_id,
-            env_file_path="test.env",
         )
 
 

python/packages/core/tests/openai/test_openai_responses_client.py

@@ -140,7 +140,6 @@ def test_init_with_default_header(openai_unit_test_env: dict[str, str]) -> None:
 def test_init_with_empty_model_id(openai_unit_test_env: dict[str, str]) -> None:
     with pytest.raises(ServiceInitializationError):
         OpenAIResponsesClient(
-            env_file_path="test.env",
         )
 
 
@@ -151,7 +150,6 @@ def test_init_with_empty_api_key(openai_unit_test_env: dict[str, str]) -> None:
     with pytest.raises(ServiceInitializationError):
         OpenAIResponsesClient(
             model_id=model_id,
-            env_file_path="test.env",
         )