feat(accounting): Kostenstellen von Eingangsbelegen jederzeit nachträglich ändern

Eigener Endpunkt und MCP-Tool ändern ausschließlich die Positionskostenstellen,
auch bei gebuchten oder einer Bankbuchung zugewiesenen Eingangsbelegen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/incomingInvoiceCostCentres.test.ts

@@ -0,0 +1,48 @@
+import assert from "node:assert/strict"
+import test from "node:test"
+import {
+    applyAccountCostCentreAssignments,
+    normalizeAccountCostCentreAssignments,
+} from "../src/modules/incoming-invoice-cost-centres.service"
+import { mcpToolMap } from "../src/mcp/registry"
+
+const CENTRE = "8f1c2b3a-4d5e-4f60-8a7b-9c0d1e2f3a4b"
+
+test("changes only requested account cost centres", () => {
+    const accounts = [
+        { account: 1, amountNet: 100, taxType: "19", costCentre: "old-1" },
+        { account: 2, amountNet: 50, taxType: "7", costCentre: "old-2" },
+    ]
+
+    assert.deepEqual(applyAccountCostCentreAssignments(accounts, [
+        { index: 1, costcentre: CENTRE },
+    ]), [
+        accounts[0],
+        { account: 2, amountNet: 50, taxType: "7", costCentre: CENTRE },
+    ])
+})
+
+test("rejects assignments for unknown invoice positions", () => {
+    assert.throws(
+        () => applyAccountCostCentreAssignments([{ account: 1 }], [{ index: 3, costcentre: null }]),
+        /Position 4.*nicht gefunden/,
+    )
+})
+
+test("validates cost centre assignments", () => {
+    assert.throws(() => normalizeAccountCostCentreAssignments([]), /Mindestens eine/)
+    assert.throws(() => normalizeAccountCostCentreAssignments([{ index: -1, costcentre: null }]), /gültigen index/)
+    assert.throws(() => normalizeAccountCostCentreAssignments([{ index: 0, costcentre: "x" }]), /Ungültige Kostenstelle/)
+    assert.throws(
+        () => normalizeAccountCostCentreAssignments([{ index: 0, costcentre: null }, { index: 0, costcentre: CENTRE }]),
+        /mehrfach/,
+    )
+    assert.deepEqual(normalizeAccountCostCentreAssignments([{ index: 0, costcentre: CENTRE }]), [{ index: 0, costcentre: CENTRE }])
+})
+
+test("registers a dedicated incoming invoice cost centre tool", () => {
+    const tool = mcpToolMap.get("accounting.incoming_invoices.cost_centres.update")
+
+    assert.deepEqual(tool?.requiredPermissions, ["accounting.incoming_invoices.write"])
+    assert.deepEqual(tool?.inputSchema.required, ["id", "accountCostCentres"])
+})