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>

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

python/samples/02-agents/observability/agent_observability.py

@@ -53,11 +53,7 @@ async def main():
         for question in questions:
             print(f"\nUser: {question}")
             print(f"{agent.name}: ", end="")
-            async for update in agent.run(
-                question,
-                session=session,
-                stream=True,
-            ):
+            async for update in agent.run(question, session=session, stream=True):
                 if update.text:
                     print(update.text, end="")