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.

1 files changed, 15 insertions, 1 deletions

diff --git a/sl0ppij/zellij-server/src/os_input_output_unix.rs b/sl0ppij/zellij-server/src/os_input_output_unix.rs
index 599bb2a..c6e1dec 100644
--- a/sl0ppij/zellij-server/src/os_input_output_unix.rs
+++ b/sl0ppij/zellij-server/src/os_input_output_unix.rs
@@ -381,7 +381,21 @@ impl UnixPtyBackend {
                     set_terminal_size_using_fd(*fd, cols, rows, width_in_pixels, height_in_pixels);
                 }
             },
-            _ => {
+            // sl0p fork: a KNOWN terminal id with no fd yet is a pane whose
+            // command has not started -- `start_suspended`, or a held pane
+            // waiting for a keypress. There is nothing to resize and nothing is
+            // wrong, so do not treat it as an error.
+            //
+            // It used to fall into the arm below and log one error per
+            // suspended pane per resize. Resizes arrive in cached bursts (see
+            // ResizeCache), so attaching to a session of suspended panes
+            // produced a storm of formatted, contexted, file-written errors on
+            // the server's hot path. Measured on a 2-vCPU box: `zellij action`
+            // against a live session failed 5/12 with two suspended panes
+            // versus 1/12 with two ordinary ones, and attached clients dropped
+            // with "lost connection to the zellij server".
+            Some(None) => {},
+            None => {
                 Err::<(), _>(anyhow!("failed to find terminal fd for id {terminal_id}"))
                     .with_context(err_context)
                     .non_fatal();