fix(cli): pin UTF-8 encoding on init-options and .extensionignore I/O (#2686)

* fix(cli): pin UTF-8 encoding on init-options and .extensionignore I/O

``Path.read_text`` / ``Path.write_text`` default to the system locale
codec, which is cp1252 / gb2312 / cp932 on Windows. Two user-facing
file paths in spec-kit were calling them without an explicit
``encoding=`` argument:

  - ``src/specify_cli/__init__.py:400,412`` —
    ``save_init_options`` / ``load_init_options`` for
    ``.specify/init-options.json``. A peer machine with a different
    default locale (or a UTF-8 Unix CI runner reading a file written on
    a cp1252 Windows host) cannot decode the file, raising
    ``UnicodeDecodeError``. ``UnicodeDecodeError`` is a subclass of
    ``ValueError`` — not ``OSError`` / ``json.JSONDecodeError`` — so
    the existing fall-back ``except`` tuple in ``load_init_options``
    also misses it and the error propagates raw to the CLI.

  - ``src/specify_cli/extensions.py:764`` — ``.extensionignore``
    pattern reader. The very next line already normalises
    backslashes "so Windows-authored files work", proving the codebase
    expects Windows authors to write this file. Multibyte UTF-8
    patterns (Chinese filenames, accented directory names) silently
    mojibake when the host locale is not UTF-8, so the patterns fail
    to match and unintended files are shipped with the extension.

The sibling integration-catalog reader at
``src/specify_cli/integrations/catalog.py:150,156,193,202,374``
already pins ``encoding="utf-8"`` everywhere. PR #2280 fixed the
symmetric PowerShell-template BOM bug. This change brings the two
remaining drifted paths in line with that precedent.

Regression tests:

  - ``tests/test_presets.py::TestInitOptions`` — parametrized non-ASCII
    round-trip (CJK, Latin-1, Greek, emoji) plus a corrupted-file case
    that asserts the existing "fall back to {}" contract still holds
    when a peer file contains bytes invalid as UTF-8.
  - ``tests/test_extensions.py::TestExtensionIgnore`` — Japanese
    (``ドキュメント/``) and Latin-1 (``café/``) ignore patterns
    correctly exclude their directories during install.

* fix(cli): wrap .extensionignore decode error and tighten UTF-8 contract

Addresses Copilot review feedback on this PR.

Three issues, three fixes:

1. ``save_init_options`` now writes JSON with ``ensure_ascii=False``.
   Without that flag, ``json.dumps`` emits ASCII-only ``\uXXXX``
   escapes, which means the ``encoding="utf-8"`` pin on the
   surrounding ``Path.write_text`` makes no observable difference for
   any value we currently write. Flipping ``ensure_ascii`` makes the
   non-ASCII bytes hit the file directly, so the encoding pin becomes
   the thing that decides between cp1252 garbage and clean UTF-8 on
   Windows. The comment above the call now describes the real reason
   instead of the previously-misleading rationale Copilot flagged.

2. ``test_save_load_round_trip_preserves_non_ascii`` was a no-op under
   the old ``ensure_ascii=True`` writer (Copilot's second comment).
   Added ``test_save_writes_real_utf8_bytes`` that asserts the on-disk
   bytes contain the UTF-8 encoding of ``café`` (``0xC3 0xA9``), not
   its JSON escape form ``é``. Removing either
   ``ensure_ascii=False`` or ``encoding="utf-8"`` from the writer now
   breaks this test — the contract is pinned.

3. ``.extensionignore`` reader wraps ``UnicodeDecodeError`` as
   ``ValidationError`` with a pointer to the offending byte
   (Copilot's third comment). Mirrors
   ``ExtensionManifest._load_yaml``'s existing handler for
   ``extension.yml``. Adds
   ``test_extensionignore_invalid_utf8_raises_validation_error``
   asserting installation aborts with the wrapped error instead of a
   raw Python traceback.

src/specify_cli/extensions.py

@@ -761,7 +761,28 @@ class ExtensionManager:
         if not ignore_file.exists():
             return None
 
-        lines: List[str] = ignore_file.read_text().splitlines()
+        # Pin UTF-8 explicitly: ``Path.read_text`` defaults to the system
+        # locale codec on Windows (cp1252 / gb2312 / cp932), which silently
+        # corrupts multibyte patterns when the file is shared across
+        # machines with different locales. The next line already
+        # normalises backslashes "so Windows-authored files work" — the
+        # codebase already expects Windows authors to write this file.
+        #
+        # A file that is not valid UTF-8 is a user-authoring mistake, so
+        # surface it as ``ValidationError`` with a pointer to the offending
+        # byte — the same pattern ``ExtensionManifest._load_yaml`` uses
+        # for ``extension.yml`` (see ``UnicodeDecodeError`` handler in
+        # this module). Without the wrap, the raw ``UnicodeDecodeError``
+        # would abort installation with a Python traceback instead of a
+        # clear message naming the file.
+        try:
+            raw = ignore_file.read_text(encoding="utf-8")
+        except UnicodeDecodeError as e:
+            raise ValidationError(
+                f".extensionignore is not valid UTF-8: {ignore_file} "
+                f"({e.reason} at byte {e.start})"
+            )
+        lines: List[str] = raw.splitlines()
 
         # Normalise backslashes in patterns so Windows-authored files work
         normalised: List[str] = []