fix(extensions): use explicit UTF-8 encoding when reading manifest YAML (#2370)

* fix(extensions): use explicit UTF-8 encoding when reading manifest YAML

On Windows, Python's open() defaults to the system locale encoding
(e.g., GBK on Chinese Windows), which causes UnicodeDecodeError when
extension.yml or preset.yml contains non-ASCII content such as Chinese
characters in description fields.

Add encoding='utf-8' to ExtensionManifest._load_yaml and
PresetManifest._load_yaml so manifests are read consistently across
platforms.

Fixes #2325

* test(extensions,presets): add UTF-8 manifest regression tests for #2325

Positive: extension.yml/preset.yml with non-ASCII (Chinese + emoji)
descriptions load correctly when written as UTF-8 bytes — fails on
Windows without explicit encoding='utf-8'.

Negative: files containing invalid UTF-8 bytes raise a clean error
(ValidationError or UnicodeDecodeError), not a silent crash.

* fix(extensions,presets): wrap I/O and decode errors as ValidationError

Address remaining Copilot concerns on #2370:

- Catch UnicodeDecodeError and OSError in both manifest loaders and
  re-raise as ValidationError / PresetValidationError so callers see a
  consistent error type, not a bare decode/IO traceback.
- Validate that PresetManifest YAML root is a mapping (extensions.py
  already had this; presets.py was missing it). Treat None as {} for
  empty-file compatibility.
- Tighten the negative regression tests to assert the specific message,
  and add a non-mapping-root test for PresetManifest matching the
  existing one for ExtensionManifest.