feat: ask for the model on the first screen of the endpoint flow

The model field was one screen too late. Reported directly: "this is where
I would have expected to see the model dropdown", on the AI Task init step
that showed only Name and the recommended checkbox.

- CONF_CHAT_MODEL moves from async_step_additional to async_step_init for
  conversation and ai_task_data, so all four endpoint types now ask on the
  first screen. stt and tts already did.
- UNSUPPORTED_MODELS is validated there too, since the recommended path
  creates the subentry from that step and would otherwise skip the check.
- model_list.default_model() picks what to preselect: RECOMMENDED_CHAT_MODEL
  only when the provider actually serves it (or discovery returned nothing),
  otherwise the first advertised model. Offering a stranger's gpt-4o-mini to
  a LiteLLM user preselects a model that fails on first use.

This is the trade I got wrong last commit. I kept the field on the second
step to avoid touching 71 ported tests, having judged them unfixable
mechanically. They were not: the 71 were ~13 functions under parametrize,
and test_subentry_switching's data is already a list of per-step dicts, so
moving a model between steps is moving a key between two dicts. Two scripted
passes moved 14 of them; 5 needed reasoning about (an error that now surfaces
on init rather than the next step, and four cases whose expected data gains
the now-always-recorded model).

async_fetch_model_ids now also catches non-OpenAIError exceptions. Found via
a ported test whose client is a MagicMock: the TypeError from awaiting it
escaped and broke the whole form. A real proxy returning a payload the SDK
cannot parse would do the same, and this runs inside a dialog with a working
free-text fallback, so nothing it raises justifies taking the form down.

Also fixes a string that was simply false on a third-party provider, again
reported from the UI: "Store requests and responses in OpenAI" / "stored by
OpenAI and visible in your OpenAI dashboard logs". It now describes the
Responses API store flag and says support varies. Swept the other 43
OpenAI-mentioning strings while there -- the rest are [%key:...%]
self-references or this fork's own action names, and no other option text
makes a claim about who stores the data.

Local: 170 passed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NxJYWgGJtCw13qUHZtG8ki

tests/test_model_list.py

@@ -73,6 +73,20 @@ async def test_fetch_survives_an_endpoint_that_will_not_list(error: Exception) -
     assert await async_fetch_model_ids(_client(error=error)) == []
 
 
+async def test_fetch_survives_a_non_openai_error() -> None:
+    """Discovery must never break the form it runs inside.
+
+    A proxy can return a payload the SDK cannot parse, or a client can be
+    wired in a way that raises something other than an OpenAIError; either way
+    the dropdown should degrade to a text box, not take the dialog down.
+    """
+    client = MagicMock()
+    client.with_options.return_value = client
+    client.models.list = MagicMock(return_value="not awaitable")
+
+    assert await async_fetch_model_ids(client) == []
+
+
 async def test_field_is_a_dropdown_that_still_accepts_a_typed_name() -> None:
     """A proxy may serve a model it does not advertise, so allow free text."""
     field = model_field(["claude-sonnet", "gpt-4o-mini"])