sl0p: stop suspended panes spamming resize errors

Attaching to a session of `start_suspended` panes could drop the client with
"your zellij client lost connection to the zellij server", and `zellij action`
against that live session failed most of the time.

Cause: `set_terminal_size` treats "no fd for this terminal id" as an error, but a
reserved-and-not-yet-started terminal is exactly what `start_suspended` (and any
held pane) is. Resizes arrive in cached bursts (ResizeCache), so every attach and
every layout change logged one formatted, contexted, file-written error PER
suspended pane, on the server's hot path.

The map distinguishes the two cases already -- `Some(None)` is reserved-without-fd
(reserve_terminal_id), `None` is genuinely unknown -- so the fix is to match them
separately and stay quiet about the former.

Measured on a 2-vCPU Debian box, `action list-tabs` against a live session:

  before   2 suspended panes  5/12 failed     2 plain panes  1/12 failed
  after    2 suspended panes  6/30 failed     2 plain panes  9/30 failed
           resize errors logged: 126+ -> 0

ie. the suspended-pane penalty is gone; what remains is a separate baseline
failure rate that affects plain sessions equally (see the known issue in
docs/roadmap.md).

zellij-server: 1499 tests still pass.

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