test(mcp): exercise books.mark_sent in the every-tool journey (P4b S5)

`inv1_every_tool_result_matches_its_output_schema` fails listing any
tool the happy-path journey never calls successfully; the new send tool
must be in it. The journey records one send on the invoice it already
created (the books never send), so its output schema is exercised on
every run like every other tool's.

crates/ledger-cli/tests/spec_mcp.rs

@@ -896,6 +896,13 @@ fn happy_path(name: &str) -> Mcp {
         json!({"kind": "invoices", "status": "all"}),
     );
     m.ok("books.document", json!({"ref": format!("invoice:{inv}")}));
+    // P4b S5: the invoice went out (the books record the send, never make it).
+    m.ok(
+        "books.mark_sent",
+        json!({"invoice": format!("invoice:{inv}"), "on": "2026-02-03",
+               "by": "Avery Quill", "recipients": ["billing@client.example"],
+               "reference": "msg-1"}),
+    );
 
     // Fixed assets.
     let registered = m.ok(