fix: responses <> chat completion input conversion (#3645)

# What does this PR do?

closes #3268
closes #3498

When resuming from previous response ID, currently we attempt to convert
from the stored responses input to chat completion messages, which is
not always possible, e.g. for tool calls where some data is lost once
converted from chat completion message to repsonses input format.

This PR stores the chat completion messages that correspond to the
_last_ call to chat completion, which is sufficient to be resumed from
in the next responses API call, where we load these saved messages and
skip conversion entirely.

Separate issue to optimize storage:
https://github.com/llamastack/llama-stack/issues/3646

## Test Plan
existing CI tests

llama_stack/apis/agents/openai_responses.py

@@ -888,6 +888,10 @@ class OpenAIResponseObjectWithInput(OpenAIResponseObject):
 
     input: list[OpenAIResponseInput]
 
+    def to_response_object(self) -> OpenAIResponseObject:
+        """Convert to OpenAIResponseObject by excluding input field."""
+        return OpenAIResponseObject(**{k: v for k, v in self.model_dump().items() if k != "input"})
+
 
 @json_schema_type
 class ListOpenAIResponseObject(BaseModel):