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>

tests/tools/test_process_registry.py

@@ -481,8 +481,8 @@ class TestSpawnEnvSanitization:
             def get_temp_dir(self):
                 return "/data/data/com.termux/files/usr/tmp"
 
-            def execute(self, command, timeout=None):
-                self.commands.append((command, timeout))
+            def execute(self, command, **kwargs):
+                self.commands.append((command, kwargs))
                 return {"output": "4321\n"}
 
         env = FakeEnv()
@@ -501,6 +501,52 @@ class TestSpawnEnvSanitization:
         assert "cat /tmp/hermes_bg_" not in bg_command
         fake_thread.start.assert_called_once()
 
+    def test_spawn_via_env_checks_returncode_when_wrapper_fails(self, registry):
+        class FakeEnv:
+            def __init__(self):
+                self.commands = []
+
+            def execute(self, command, **kwargs):
+                self.commands.append((command, kwargs))
+                return {"output": "syntax error", "returncode": 2}
+
+        env = FakeEnv()
+        fake_thread = MagicMock()
+
+        with patch("tools.process_registry.threading.Thread", return_value=fake_thread), \
+            patch.object(registry, "_write_checkpoint"):
+            session = registry.spawn_via_env(env, "echo hello")
+
+        assert session.exited is True
+        assert session.exit_code == 2
+        assert session.pid is None
+        assert session.output_buffer == "syntax error"
+        fake_thread.start.assert_not_called()
+        # A failed launch must not be exposed as a running/tracked session.
+        assert session.id not in registry._running
+
+    def test_spawn_via_env_disables_rewrite_for_bg_wrapper(self, registry):
+        class FakeEnv:
+            def __init__(self):
+                self.commands = []
+
+            def get_temp_dir(self):
+                return "/tmp"
+
+            def execute(self, command, **kwargs):
+                self.commands.append((command, kwargs))
+                return {"output": "4321\n", "returncode": 0}
+
+        env = FakeEnv()
+        fake_thread = MagicMock()
+
+        with patch("tools.process_registry.threading.Thread", return_value=fake_thread), \
+            patch.object(registry, "_write_checkpoint"):
+            registry.spawn_via_env(env, "echo hello")
+
+        args, kwargs = env.commands[0]
+        assert kwargs.get("rewrite_compound_background") is False
+
     def test_env_poller_quotes_temp_paths_with_spaces(self, registry):
         session = _make_session(sid="proc_space")
         session.exited = False
@@ -514,8 +560,8 @@ class TestSpawnEnvSanitization:
                     {"output": "0\n"},
                 ])
 
-            def execute(self, command, timeout=None):
-                self.commands.append((command, timeout))
+            def execute(self, command, **kwargs):
+                self.commands.append((command, kwargs))
                 return next(self._responses)
 
         env = FakeEnv()