test(mcp): exercise the write-off in the happy path (P4b S7)

spec_mcp's inv1 walks every tool once and fails on any the books never
answered: the happy path now creates an invoice that will not be paid,
uploads the collection letter, and writes it off with the evidence in the
same call, so books.write_off_invoice's output schema is checked like
every other tool's.

crates/ledger-cli/tests/spec_mcp.rs

@@ -896,6 +896,20 @@ fn happy_path(name: &str) -> Mcp {
         json!({"kind": "invoices", "status": "all"}),
     );
     m.ok("books.document", json!({"ref": format!("invoice:{inv}")}));
+    // P4b S7: a bad debt, its evidence uploaded and filed with it.
+    let bad = m.ok(
+        "books.create_invoice",
+        json!({"party_id": customer, "date": "2026-02-03", "due_date": "2026-03-03",
+               "lines": [{"description": "Consulting, never paid", "amount": "400.00"}]}),
+    )["id"]
+        .as_u64()
+        .unwrap();
+    let letter = m.upload(b"Final demand", "letter.txt", "text/plain", "document");
+    m.ok(
+        "books.write_off_invoice",
+        json!({"invoice": bad, "date": "2026-03-01", "reason": "collection exhausted",
+               "upload_id": letter, "confirmed": true}),
+    );
 
     // Fixed assets.
     let registered = m.ok(