fix(codex): omit tools key from Codex Responses kwargs when no tools registered

Salvages the transport-side fix from #32911 (@xxxigm). Closes #32892.

The openai SDK's responses.stream() / responses.parse() eagerly call
_make_tools(tools), which iterates tools without a None guard. Passing
tools=None raises TypeError: 'NoneType' object is not iterable before
any HTTP request is issued (openai==2.24.0).

PR #33042 already removed responses.stream() from our own Codex call
paths, so the specific iteration crash inside _make_tools is no longer
on the hot path. But the right API contract is to omit tools entirely
when there are no functions to expose — passing tools=None to the
backend is semantically wrong regardless of the SDK's iteration
behavior, and we'd hit it again on any future code path that hasn't
migrated off responses.stream().

This applies the transport-level part of @xxxigm's fix: move
'tools': response_tools into the if response_tools: branch so the
key is omitted when there are no tools, just like tool_choice and
parallel_tool_calls already are. Skips the run_agent.py-side
_strip_sdk_none_iterables helper from their PR — that path is now
obsolete because the SDK helper that needed defending is gone.

Tests
- tests/run_agent/test_codex_no_tools_nonetype.py: 6 tests trimmed
  from @xxxigm's original 13-test file. Drops the obsolete tests for
  _strip_sdk_none_iterables and _RecordingResponsesStream (helpers
  that don't exist on main anymore), keeps the transport behavior
  tests + the SDK contract sanity check that ensures we notice if
  upstream ever fixes _make_tools(None).
- 6/6 passing locally.

Co-authored-by: xxxigm <tuancanhnguyen706@gmail.com>

agent/transports/codex.py

@@ -128,6 +128,14 @@ class ResponsesApiTransport(ProviderTransport):
         reasoning_effort = _effort_clamp.get(reasoning_effort, reasoning_effort)
 
         response_tools = _responses_tools(tools)
+        # ``tools`` MUST be omitted entirely when there are no functions to
+        # expose: the openai SDK's ``responses.stream()`` / ``responses.parse()``
+        # eagerly call ``_make_tools(tools)`` which does ``for tool in tools``
+        # without a None guard, so passing ``tools=None`` raises
+        # ``TypeError: 'NoneType' object is not iterable`` before any HTTP
+        # request is issued (openai==2.24.0).  Reported for the
+        # ``openai-codex`` / ``gpt-5.5`` combo on chatgpt.com/backend-api/codex
+        # (#32892) when the agent runs without external tools registered.
         kwargs = {
             "model": model,
             "instructions": instructions,
@@ -137,10 +145,10 @@ class ResponsesApiTransport(ProviderTransport):
                 replay_encrypted_reasoning=replay_encrypted_reasoning,
                 current_issuer_kind=issuer_kind,
             ),
-            "tools": response_tools,
             "store": False,
         }
         if response_tools:
+            kwargs["tools"] = response_tools
             kwargs["tool_choice"] = "auto"
             kwargs["parallel_tool_calls"] = True