fix(units): patch synonyms as item-level add/delete operations

[prasad-albert]

Summary

• Updating a unit's synonyms previously sent a single whole-list update op, which the backend rejects — it has no update operation for Synonyms (only item-level add/delete, each with a scalar value), and it rejects more than one operation on the same attribute in a single request.
• synonyms changes are now diffed into per-item add/delete operations, and each Synonyms operation is sent as its own PATCH request (other attributes like symbol/category still go in one request).
• Folded synonyms coverage into the existing test_unit_crud (no new fixture/seeding) — adds and removes a synonym and asserts the result. Verified end-to-end against app.albertinvent.com (full test_units.py suite: 8 passed).

[claude]

Code Review

Issues Found

• [src/albert/collections/units.py:121] update() is missing @validate_call. Every other public collection method in this file has it (get_by_id, get_by_ids, delete), and the method accepts a Unit argument that should be validated at the boundary.

Summary

1 issue found. Low-severity — the fix logic is correct; the missing decorator is the only gap.

[prasad-albert]

Unset synonyms will delete all existing synonyms

_generate_unit_patch_payload reads updated.synonyms or [] without checking model_fields_set. If a caller builds a partial update object without setting synonyms:

unit = Unit(id="x", symbol="y")   # synonyms never set → not in model_fields_set
client.units.update(unit=unit)

updated.synonyms or [] resolves to [], so existing_synonyms - [] emits a DELETE for every existing synonym. Same unset-vs-empty bug fixed in base.py / generate_adv_patch_payload in #561.

One-liner fix — add this right after stripping the old Synonyms datum:

payload.data = [d for d in payload.data if d.attribute != "Synonyms"]
if "synonyms" not in updated.model_fields_set:
    return payload   # caller never set synonyms → leave them untouched

Explicit synonyms=[] still lands in model_fields_set so intentional clears are preserved.