Extract shared deserialize helpers on Device

[frenckatron]

What

Extract three shared helpers (_filter_effects, _build_palette_dict,
_split_presets_playlists) on Device and call them from both
__pre_deserialize__ and update_from_dict.

Why

The two methods carried near-identical logic for effects filtering,
palette synthesis, and preset/playlist splitting. Any fix landing in one
risked silently diverging from the other — a real dual-maintenance
hazard. This is the refactor specced for src/wled/models.py:865-1011.

How

• Helpers live next to the existing _build_custom_palettes /
  _build_usermod_palettes statics. _build_palette_dict is a
  @classmethod because it composes the other two; the rest are
  @staticmethod.
• All helpers return raw dict[str, Any]. The pre-deserialize path
  hands the dicts to mashumaro directly; update_from_dict wraps them
  with Effect(**…), Palette(**…), and Preset.from_dict /
  Playlist.from_dict. One source of truth, two output shapes.
• Behavior preserved: RSVD skip, null-palette fallback for
  less-capable devices, defensive _presets.copy(), and pop(0, None)
  on both presets and playlists. Version-check / WLEDUnsupportedVersionError
  stays inside __pre_deserialize__ since it's deserialization-specific.

Testing

• Full suite: 222 passed (37 snapshots passed), no behavior change.
• ruff check src/wled/models.py clean.

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Quality Report

Changes: 1 file changed, 118 insertions(+), 72 deletions(-)

Code scan: clean

Tests: failed (command not found)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch frenckatron/refactor-device-deserialize-helpers

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