fix(aux): default Codex reasoning effort to medium when extra_body.reasoning.effort is falsy

auxiliary.<task>.extra_body.reasoning, but the new translation path in
_CodexCompletionsAdapter.create() reads the effort with
``reasoning_cfg.get("effort", "medium")``.  That returns the configured
value verbatim when the key is present, so ``effort: null`` /
``effort: ""`` (both common YAML shapes) flow through as
``{"effort": null, "summary": "auto"}`` and Codex rejects the request
with "Invalid value for parameter ``reasoning.effort``".

agent/transports/codex.py::build_kwargs() — which the new adapter is
documented to mirror — uses a truthy check (``elif
reasoning_config.get("effort"):``) so the same falsy values keep the
"medium" default.  Switch the auxiliary adapter to the same
``or "medium"`` truthy form so identical config produces identical
requests on both paths.

- [x] Two new regression tests cover ``effort: None`` and
  ``effort: ""`` and assert the request goes out as
  ``{"effort": "medium", "summary": "auto"}``.
- [x] Old behaviour fails the new tests (``{'effort': None} !=
  {'effort': 'medium'}``); fixed behaviour passes all 11 tests in the
  ``TestCodexAdapterReasoningTranslation`` class.
- [x] Adjacent suites green: ``tests/agent/test_auxiliary_client.py``
  (108 passed) and ``tests/agent/transports/test_codex_transport.py +
  test_chat_completions.py`` (73 passed).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agent/auxiliary_client.py

@@ -567,7 +567,12 @@ class _CodexCompletionsAdapter:
                     # API allows it.
                     pass
                 else:
-                    effort = reasoning_cfg.get("effort", "medium")
+                    # Truthy-only check mirrors agent/transports/codex.py
+                    # build_kwargs(): falsy values (None, "", 0) fall back
+                    # to the default rather than being forwarded to the
+                    # Codex backend, which rejects e.g. {"effort": null}
+                    # with a 400.
+                    effort = reasoning_cfg.get("effort") or "medium"
                     # Codex backend rejects "minimal"; clamp to "low" to
                     # match the main-agent Codex transport behavior.
                     if effort == "minimal":