Stop reporting stale replies as successful conversions

[gmile]

When soffice dies mid-conversion, URP.Pool.convert/3 returns {:ok, "<some interface OID>"} instead of an error, and the real failure is lost.

Bridge.recv_reply/1 and Bridge.send_frame/2 rescue socket errors into conn.error but leave conn.reply alone, so it still holds whatever the previous call parsed. In the middle of store_document_write/2 that is an interface OID from queryInterface. The has_result guard only checked the shape of the reply (is_binary(result) or result == :ok), which a stale OID satisfies just as well as real output bytes.

Callers see it as a case clause error on a value that looks nothing like what they asked for:

** (CaseClauseError) no case clause matching:

    {:ok, "57ee30e3f490;gcc3[0];46d08adf17bf43b189511ea4ed49b46f"}

    (urp 0.10.2) lib/urp.ex:370: URP.do_convert/2

The fix is to capture conn.error before Bridge.cleanup/1 (which only ever appends to it) and trust the reply only when the conversion itself reported no error. A cleanup failure after a successful conversion still returns the output, as before. a45f645 fixed the sibling case where a bootstrap desktop OID leaked out of an early failure, but the same reply-shape guard is still what decides here.

The first commit is a pure extraction of the decision into checkout_outcome/4 so it can be tested without a live soffice; the second is the behaviour change. Both new tests fail on the first commit and pass on the second. The third commit bumps mint to clear an unrelated mix hex.audit advisory that was turning CI red.

To reproduce against a real soffice, pass a recv_timeout short enough to die mid-pipeline but long enough to clear the first few calls:

URP.convert("sample.docx", filter: "writer_pdf_Export", output: "out.pdf", recv_timeout: 5)

One thing this does not address: the surfaced message is currently "no match of right hand side value: {:error, :timeout}", because recv_exact/3 matches on {:ok, payload} = :gen_tcp.recv(...). Worth a proper raise, but leaving it out of this change.