fix(extensions): start fresh on a non-UTF-8 extension registry

[Noor-ul-ain001]

Problem

ExtensionRegistry._load() treats a corrupted registry as "start fresh", but only for two exception types:

except (json.JSONDecodeError, FileNotFoundError):
    # Corrupted or missing registry, start fresh

A .specify/extensions/.registry file containing invalid UTF-8 bytes raises UnicodeDecodeError from the text-mode read before JSON parsing begins, so it escapes that clause. Because the registry is loaded in __init__, the bare traceback breaks every extension command on the project:

$ specify extension list
...
  File ".../specify_cli/extensions/__init__.py", line 655, in _load
    data = json.load(f)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte

ExtensionRegistry is also constructed from events.py and from two places in presets/__init__.py, so the crash reaches beyond the extension command group.

Fix

Catch UnicodeDecodeError in the same clause — undecodable bytes are the same corruption class as unparseable JSON, only the exception type differs.

OSError stays uncaught on purpose: the data may be intact on disk, and starting fresh would let a later _save() wipe it (same fail-closed reasoning as the workflow catalog cache loader).

After the fix the command degrades gracefully:

$ specify extension list
No extensions installed.

Relationship to #3955

This is the exact twin of the PresetRegistry._load() fix merged in #3955. The two registries are parallel implementations of the same pattern and only the preset side was corrected there; this applies the identical one-line change plus rationale comment to the extension side.

Notably, _get_installed_sibling_ids() already documented and worked around this precise gap at its own call site:

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.

That comment is updated to reflect that _load() now handles the case itself; the local catch is kept as belt-and-braces against regression.

Testing

Added test_load_starts_fresh_for_non_utf8_registry, mirroring the #3955 preset test: writes undecodable bytes to .registry, then asserts the registry starts fresh and list() / is_installed() behave.

Verified the test genuinely covers the fix — with src/specify_cli/extensions/__init__.py reverted it fails with the original UnicodeDecodeError, and passes with the fix applied.

pytest tests/test_extensions.py → 489 passed. pytest tests/test_presets.py tests/integrations/test_events.py → 678 passed. The only failures in either run are pre-existing symlink tests that require Windows elevation (WinError 1314) and fail identically on a clean checkout. ruff check passes on both changed files.

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