Update Vat Rate

PUT /companies/{id}/accounting-settings/vat-rates/{rate_id} body (admin).

Three independent edits, each applied only when the key is present in the request body (model_fields_set) — so the historical {"active": …} call is unchanged and cannot accidentally clear a link:

• active — the manual activation toggle: sets active and pins the row (active_source="manual") so document/bexio writers never clobber it.

• revenue_account_code (ZET-210) — the revenue account of the ESTV activity a settlement rate (kind='saldo'/'saldo_neu') settles. This is the only path that can clear the link (send an explicit null): no automatic writer ever sets it, so nothing else could. Refused (422) on a non-settlement row, on an account that is not an income, non-contra account of the company's chart, and when another settlement rate already claims that account.

• activity_id (F1414) — the partner's ESTV activity ID of a settlement rate: the five-character activityID the ESTV issued for the approved activity (shown under «Abrechnungsmodalitäten» in «MWST abrechnen»), under which a Saldo/Pauschal return from 2025 declares the rate's turnover (eCH-0217 V2). null/blank clears it. Refused (422) on a non-settlement row, on anything but five letters/digits, and on the ESTV's technical IDs T0001–T0020 (Zeno applies those itself to pre-2025 supplies).

• granted_from / valid_to (F2418 (6), F2411) — a settlement rate's ESTV grant date and end date, each applied only when present (an explicit null clears). With them successive editions' rates of one activity — e.g. 5.9 % to 31.12.2023 and 6.2 % granted from 1.1.2024 — each settle their own supplies, and may link the same revenue account as long as their windows do not overlap. Refused on a non-settlement row, and when the window is inverted.

An empty body is refused rather than treated as a silent 200 no-op. A write that turns an effective → SSS entry the books already show ineligible is refused (F2418 (3): the entry test runs from the rate writer too).

A declared VAT rate row.

kind stays a free string (it also carries bexio type strings), but 'saldo'/'saldo_neu' rows are settlement rates (the ESTV-approved Saldosteuersatz — a turnover-settlement rate, never a line rate); those rows may link the revenue account of their SSS activity.