Allow label edits to round-trip the path fields

[level09]

Label.to_dict() returns path / path_ar (added in #369), and the admin edit form PUTs that payload straight back. LabelValidationModel is strict, so every label edit fails with:

[path]: Extra field not allowed
[path_ar]: Extra field not allowed

Adds both to the same discard list that already carries id, order, updated_at, parent_label_id and children.

Reported by @apodacaduron on #378.

Tests

• New regression test PUTs a label's own to_dict() back and expects 200 (fails with 400 without the fix).
• uv run pytest: 869 passed, 4 skipped.

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[level09]

Folded into #378 instead, same author, and that PR is already approved.