Move child exit handling logic to apprt thread

Fixes #7500
Supersedes #7582

This commit moves the child exit handling logic from the IO thead to the
apprt thread. The IO thread now only sends a `child_exited` message to
the apprt thread with metadata about the exit conditions (exit code,
runtime).

From there, the apprt thread can handle the exit situation however is
necessary. This commit doesn't change the behavior but it does fix the
issue #7500. The behavior is: exit immediately, show abnormal exit
message, wait for user input, etc.

This also gets us closer to #7649.

src/termio/Thread.zig

@@ -311,7 +311,6 @@ fn drainMailbox(
             .jump_to_prompt => |v| try io.jumpToPrompt(v),
             .start_synchronized_output => self.startSynchronizedOutput(cb),
             .linefeed_mode => |v| self.flags.linefeed_mode = v,
-            .child_exited_abnormally => |v| try io.childExitedAbnormally(v.exit_code, v.runtime_ms),
             .focused => |v| try io.focusGained(data, v),
             .write_small => |v| try io.queueWrite(
                 data,