docs: update workaround rationale now that _load handles UnicodeDecodeError

Review follow-up: _sibling_extension_ids and
test_non_utf8_registry_does_not_crash documented the pre-fix _load()
contract; reword both as defense-in-depth so the rationale stays true.

Assisted-by: GitHub Copilot (model: claude-fable-5, autonomous)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: marcelsafin

src/specify_cli/extensions/__init__.py

@@ -4259,11 +4259,11 @@ def _sibling_extension_ids(self) -> list[str]:
         Returns an empty list if the registry is missing or corrupted
         (fresh project, ad-hoc test harness) so ``_get_env_config`` degrades
         to its pre-fix behaviour rather than crashing. ``UnicodeError`` is
-        caught alongside ``OSError`` because ``ExtensionRegistry._load()``
-        opens the file in text mode and only handles ``JSONDecodeError`` /
-        ``FileNotFoundError``, so a registry file with non-UTF-8 bytes would
-        otherwise surface a ``UnicodeDecodeError`` here and break *every*
-        config read instead of degrading gracefully.
+        caught alongside ``OSError`` as defense in depth:
+        ``ExtensionRegistry._load()`` now starts fresh on a non-UTF-8
+        registry itself, but this scan must degrade gracefully even if that
+        contract regresses, because a failure here would break *every*
+        config read.
 
         Used by ``_get_env_config`` to detect env vars whose remainder claims
         a longer, sibling-owned prefix (e.g. ``SPECKIT_GIT_HOOKS_URL`` is

tests/test_extensions.py

@@ -10472,10 +10472,10 @@ def test_config_only_leftover_not_treated_as_sibling(self, tmp_path, monkeypatch
     def test_non_utf8_registry_does_not_crash(self, tmp_path, monkeypatch):
         """A registry file with invalid text encoding must NOT propagate
         ``UnicodeDecodeError`` out of the sibling scan and abort every
-        config read. ``ExtensionRegistry._load()`` catches ``JSONDecodeError``
-        / ``FileNotFoundError`` only, so ``_sibling_extension_ids`` must
-        additionally swallow ``UnicodeError`` and degrade to the documented
-        pre-fix behaviour.
+        config read. ``ExtensionRegistry._load()`` now starts fresh on a
+        non-UTF-8 registry itself, but ``_sibling_extension_ids`` keeps
+        swallowing ``UnicodeError`` as defense in depth so a regression of
+        that contract still degrades to the documented pre-fix behaviour.
         """
         extensions_dir = tmp_path / ".specify" / "extensions"
         extensions_dir.mkdir(parents=True)