fix(tools): don't compound-rewrite spawn_via_env background wrappers

Background tasks on non-local backends (SSH/Docker/Modal/Daytona/Singularity)
go through `ProcessRegistry.spawn_via_env`, which builds a hand-crafted,
shell-safe wrapper:

    mkdir -p T && ( nohup bash -lc CMD > LOG 2>&1; rc=$?; ... ) & echo $! > PID && cat PID

`BaseEnvironment.execute()` unconditionally ran `_rewrite_compound_background`
on every command, including this wrapper. The rewrite (meant to defuse the
`A && B &` subshell-wait trap for user commands) turns `( ... ) & echo $!` into
`{ ( ... ) & } echo $!` — note `} echo` with no separator, which is a bash
syntax error. The wrapper then never produces a PID, the redirected output file
is never created, and the agent sees an immediate exit code -1. This breaks
*every* background launch on a non-local backend (e.g. a simple
count-and-redirect script over SSH), not just edge cases.

Fix:
- Add `rewrite_compound_background: bool = True` to `BaseEnvironment.execute()`
  (and the `BaseModalExecutionEnvironment` override, which accepts and ignores
  it). Default preserves existing behavior; the user foreground terminal path
  still rewrites.
- `spawn_via_env` passes `rewrite_compound_background=False` so its already
  shell-safe wrapper is left intact.
- Treat a wrapper that produces no PID as a failed launch (mark the session
  exited with a real exit code instead of exposing a fake running session), and
  don't register/checkpoint a session that never started.

Verified empirically: with the rewrite skipped, the wrapper is valid bash,
launches the process, captures the PID, and writes the log/pid/exit files; the
old rewritten form fails `bash -n` with a syntax error.

Based on #33756 by @CharZhou (extracted from a multi-feature branch; the
unrelated image_gen / docker-media changes are not included here).

Co-authored-by: CharZhou <17255546+CharZhou@users.noreply.github.com>

tools/environments/modal_utils.py

@@ -79,7 +79,12 @@ class BaseModalExecutionEnvironment(BaseEnvironment):
         *,
         timeout: int | None = None,
         stdin_data: str | None = None,
+        rewrite_compound_background: bool = True,
     ) -> dict:
+        # Managed/remote modal transports execute commands via explicit transport
+        # and do not rely on shell background rewriters. Keep parameter for
+        # compatibility with BaseEnvironment callers.
+        _ = rewrite_compound_background
         self._before_execute()
         prepared = self._prepare_modal_exec(
             command,