Python: Fix OpenAI Responses streaming to propagate created_at from final response.completed event (#5382)

* Fix streaming response losing created_at from response.completed event (#5347)

The streaming path in _parse_chunk_from_openai did not extract created_at
from the response.completed event, unlike the non-streaming path in
_parse_responses_response. This caused durabletask persistence warnings
when created_at was None.

Extract created_at in the response.completed case and pass it to the
returned ChatResponseUpdate.

Also fix pre-existing pyright errors for optional orjson import in sample
files.

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

* Fix orjson import suppression to use pyright instead of mypy (#5347)

Replace `# type: ignore[import-not-found]` with
`# pyright: ignore[reportMissingImports]` on optional orjson imports
in conversation sample files, matching the repo's Pyright strict
configuration.

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

---------

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

python/packages/openai/agent_framework_openai/_chat_client.py

@@ -2028,6 +2028,7 @@ class RawOpenAIChatClient(  # type: ignore[misc]
         local_shell_tool_name = self._get_local_shell_tool_name(options.get("tools"))
         conversation_id: str | None = None
         response_id: str | None = None
+        created_at: str | None = None
         continuation_token: OpenAIContinuationToken | None = None
         model = self.model
         match event.type:
@@ -2209,6 +2210,9 @@ class RawOpenAIChatClient(  # type: ignore[misc]
                 response_id = event.response.id
                 conversation_id = self._get_conversation_id(event.response, options.get("store"))
                 model = event.response.model
+                created_at = datetime.fromtimestamp(event.response.created_at, tz=timezone.utc).strftime(
+                    "%Y-%m-%dT%H:%M:%S.%fZ"
+                )
                 if event.response.usage:
                     usage = self._parse_usage_from_openai(event.response.usage)
                     if usage:
@@ -2589,6 +2593,7 @@ class RawOpenAIChatClient(  # type: ignore[misc]
             response_id=response_id,
             role="assistant",
             model=model,
+            created_at=created_at,
             continuation_token=continuation_token,
             additional_properties=metadata,
             raw_representation=event,