feat(cliprdr): add CliprdrBackend::on_format_list_response(ok) hook

[clintcan]

Add a non-breaking backend hook that fires when the remote responds to a FormatList we sent (an outbound advertise of our own clipboard contents):

fn on_format_list_response(&mut self, _ok: bool) {}

Called from handle_format_list_response on both branches:

• Ok → on_format_list_response(true)
• Fail → on_format_list_response(false) (after Cliprdr has cleared local_file_list per MS-RDPECLIP 3.1.5.2.4)

Default impl is a no-op, so this is non-breaking for existing backends.

Why

Without this signal, a backend has no way to learn whether an outbound FormatList was accepted or rejected. That's a real problem because the Fail branch silently wipes local_file_list / local_file_list_format_id per the spec — so a rejected advertise breaks the paste with no recovery and no way to detect it.

Worse: a naive timed-retry strategy can actively wipe an already-Ok state by sending a follow-up advertise that happens to be rejected. Observed live (without the hook, so no way to stop):

advertise #1 → Fail
advertise #2 (retry) → Ok (file present on remote clipboard)
advertise #3 (retry) → Fail   ← wiped the accepted state from #2
                                paste then fails

With the hook, a backend can retry on Fail and stop the moment Ok arrives, preserving the success.

Use case / verification

Used in macrdp (a Rust RDP server for macOS) to implement reliable Mac→Windows file copy. mstsc commonly rejects the first FormatList right after an in-session Cmd-C (it's still processing the input) and accepts one ~0.5–1s later. With this hook, the macrdp poller's retry loop fires retries on Fail and stops on Ok (via a shared generation/locked-gen pair), fixing the silent-paste-failure case while still recovering from a transient initial Fail. Verified end-to-end with real mstsc.

Related

Pairs naturally with the upcoming PreferredDropEffect and FD_PROGRESSUI PRs to deliver the full "Mac→Windows clipboard file paste with native progress dialog" UX, but is independently useful and can be merged on its own.