fix: backfill base_url on migration so pre-base_url entries still load

async_setup_entry does entry.data[CONF_BASE_URL] as a hard index, but
base_url is this fork's own addition: an entry created by a build from
before the base_url commits (48195c9 era, version 2 minor 7) has no such
key. On restart, setup raises KeyError: 'base_url' and the integration
fails to load -- reproduced in test first, where async_setup returns
False with that KeyError in the log.

Fixed with a migration step rather than a defensive .get(), matching the
pattern the rest of async_migrate_entry already uses, so the invariant
"a loaded entry has base_url" holds for every caller instead of each
call site guarding itself:

- new step 2.7 -> 2.8 backfills DEFAULT_BASE_URL via dict.setdefault, so
  an entry that already has a base_url keeps its own endpoint and only a
  missing one is filled. An install from before this feature was
  implicitly using api.openai.com, which is exactly DEFAULT_BASE_URL, so
  the backfill preserves existing behavior rather than repointing anyone.
- config_flow MINOR_VERSION 7 -> 8, otherwise the step never runs.

test_client_built_with_base_url now also covers the preserve path, since
its entry starts at minor 7 with a base_url set and must still reach the
client unchanged after migrating.

Also updated test_form's `minor_version == 7` assertion to 8; it pins
the version new entries are created at, so the bump is expected. The
reauth test's MockConfigEntry stays at minor 7 on purpose -- it stands
in for an already-existing entry.

Local: 138 passed.

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

tests/test_config_flow.py

@@ -122,7 +122,7 @@ async def test_form(hass: HomeAssistant) -> None:
         },
     ]
     assert result2["version"] == 2
-    assert result2["minor_version"] == 7
+    assert result2["minor_version"] == 8
     assert len(mock_setup_entry.mock_calls) == 1