terminal: write() throws "Terminal is closed" when converting data closes the terminal - #47032

Reproduction (bun 1.4.3-canary and main at 449877f, Linux x64, deterministic):

const t = new Bun.Terminal({ cols: 80, rows: 24 });
const s = new String("hello");
s.toString = () => {
  t.close();
  return "hello";
};
console.log(t.write(s)); // 5. With this PR: throws "Terminal is closed"
console.log(t.closed); // true
t.write("x"); // throws "Terminal is closed"

The same happens with a String subclass, with Symbol.toPrimitive or valueOf as the hook, and with [Symbol.asyncDispose]() as the closer. A hook that closes and returns "" makes write() return 0.

Outcome matrix. 23 argument kinds, each on an open and on an already closed terminal, on debug and release builds, without and with this change.

• 4 of 46 rows change. Each is a hook that closes an open terminal: write() returned 5, 0, 4 or 5000 and now throws.
• All 23 rows for a closed terminal throw Terminal is closed with 0 hook calls on both builds.
• A hook that closes the terminal and then throws still propagates its own exception.

Outside the matrix: after PTY EOF (closed is false) write("x") returns 1 and write("") returns 0 on both builds.

Measurements. Release builds (ThinLTO) of main at 449877f, without and with this change, Linux x64. perf, valgrind and strace are not installed in this environment, so the counts come from gdb.

• Accept path, non-empty write to an open terminal: +2 instructions, 1 flags test with mask 0x21 (1 with mask 0x20 on main) (objdump).
• Instructions per write(): '' 351 to 357, 'x' 579 to 581, Uint8Array(1) 518 to 520, 16 non-ASCII characters 1329 to 1331, String object 1097 to 1099, 'x' after PTY EOF 390 to 392 (gdb stepi, JIT off, call 41 of each kind, 3 runs each, all equal).
• Release binary: the write function grows by 38 bytes (4112 to 4150). Total text does not change (nm -S, size).
• mi_malloc calls in 1000 write() calls: 'x' 1 to 1, 16 non-ASCII characters 1001 to 1001, 64 KB Uint8Array 1 to 1. A closing hook: 2057 to 5094, the thrown error. A write to a closed terminal: 3035 on both builds. mi_realloc calls: 7 to 7, 1008 to 1007 and 11 to 11 (gdb breakpoint counts, main thread, inside the call).
• Syscalls for 1000 buffered writes and their drain: write 2 to 2, epoll_ctl 1002 to 1002. A closing-hook call issues 0 write(2) of its payload on both builds (gdb catch syscall).
• A separate CLOSED test after the conversion, as in ServerWebSocket::send_message, was also measured: 3 more instructions on every path and 146 bytes.

Sites with the same order that this PR does not change

• The termios flag setters (Terminal.rs:1321): valueOf can close the terminal after the closed test. The result equals the documented no-op on a closed terminal. terminal: throw when a termios flag assignment fails and clear its mordant baseline entry #39162 and terminal: throw when a termios flag is set to a value that is not a uint32 #42589 rewrite that function.
• Bun.WebView: webview: re-check that the view is open after argument conversion, in the shared pre-dispatch check #42160.
• Bun.listen().reload() (src/runtime/socket/Listener.rs:187): an option getter that stops the listener makes reload() return, where a stopped listener throws Listener is closed.
• Host functions in other subsystems are not part of this PR.
• Checked and not affected: resize() (it accepts only numbers), setRawMode() (ToBoolean runs no user code) and Bun.spawn({ terminal }) (no user code runs between the closed test and the spawn).

#46789 rewrites the type of the data argument and keeps this order on purpose. Its schema comment says: "toString of a String object runs once the terminal is known to be open". At its head 68867e8 the lines after the conversion are unchanged, so the symptom is still there. This commit applies to that head without a conflict (git merge-tree --merge-base).

Platforms. Built and tested on Linux x64 only. The edited lines have no cfg, and close() sets CLOSED before any platform code. The PTY EOF test does not run on Windows, where the child's exit leaves the writer open.

Docs and types. No change. bun.d.ts does not describe the throw on a closed terminal.