Python: Fix FoundryAgent stripping model from PromptAgent requests (#5526)

* Fix FoundryAgent stripping model from PromptAgent requests

Move run_options.pop('model', None) inside the _uses_foundry_agent_session()
conditional so that model is only stripped for hosted agent sessions (where
the server manages the model) and preserved for PromptAgent requests that
require it in the Responses API call.

Fixes #5525

* test: add coverage for resp_* continuation preserving model

Adds test_raw_foundry_agent_chat_client_prepare_options_preserves_model_for_resp_continuation
to explicitly verify that HostedAgent v1 / v2-no-session paths (where conversation_id
starts with resp_) preserve model and previous_response_id without triggering the
hosted-session gate.

---------

Co-authored-by: Benke Qu <bequ@microsoft.com>
Co-authored-by: Evan Mattson <35585003+moonbox3@users.noreply.github.com>

python/packages/foundry/agent_framework_foundry/_agent.py

@@ -351,6 +351,7 @@ class RawFoundryAgentChatClient(  # type: ignore[misc]
         if _uses_foundry_agent_session(conversation_id):
             run_options.pop("previous_response_id", None)
             run_options.pop("conversation", None)
+            run_options.pop("model", None)
             extra_body["agent_session_id"] = conversation_id
         # Non-preview Prompt/Hosted Agent calls need agent_reference in the request body to
         # tell the Responses API which Foundry agent (and version) is in use, since ``model``
@@ -366,7 +367,6 @@ class RawFoundryAgentChatClient(  # type: ignore[misc]
         # Strip tools from request body - Foundry API rejects requests with both
         # agent endpoint and tools present. FunctionTools are invoked client-side
         # by the function invocation layer, not sent to the service.
-        run_options.pop("model", None)
         if not self.allow_preview:
             run_options.pop("tools", None)
             run_options.pop("tool_choice", None)