feat: execute_code runs on remote terminal backends (#5088)

* feat: execute_code runs on remote terminal backends (Docker/SSH/Modal/Daytona/Singularity)

When TERMINAL_ENV is not 'local', execute_code now ships the script to
the remote environment and runs it there via the terminal backend --
the same container/sandbox/SSH session used by terminal() and file tools.

Architecture:
- Local backend: unchanged (UDS RPC, subprocess.Popen)
- Remote backends: file-based RPC via execute_oneshot() polling
  - Script writes request files, parent polls and dispatches tool calls
  - Responses written atomically (tmp + rename) via base64/stdin
  - execute_oneshot() bypasses persistent shell lock for concurrency

Changes:
- tools/environments/base.py: add execute_oneshot() (delegates to execute())
- tools/environments/persistent_shell.py: override execute_oneshot() to
  bypass _shell_lock via _execute_oneshot(), enabling concurrent polling
- tools/code_execution_tool.py: add file-based transport to
  generate_hermes_tools_module(), _execute_remote() with full env
  get-or-create, file shipping, RPC poll loop, output post-processing

* fix: use _get_env_config() instead of raw TERMINAL_ENV env var

Read terminal backend type through the canonical config resolution
path (terminal_tool._get_env_config) instead of os.getenv directly.

* fix: use echo piping instead of stdin_data for base64 writes

Modal doesn't reliably deliver stdin_data to chained commands
(base64 -d > file && mv), producing 0-byte files. Switch to
echo 'base64' | base64 -d which works on all backends.

Verified E2E on both Docker and Modal.

tools/environments/base.py

@@ -91,6 +91,19 @@ class BaseEnvironment(ABC):
             kw["stdin"] = subprocess.DEVNULL
         return kw
 
+    def execute_oneshot(self, command: str, cwd: str = "", *,
+                        timeout: int | None = None,
+                        stdin_data: str | None = None) -> dict:
+        """Execute a command bypassing any persistent shell.
+
+        Safe for concurrent use alongside a long-running execute() call.
+        Backends that maintain a persistent shell (SSH, Local) override this
+        to route through their oneshot path, avoiding the shell lock.
+        Non-persistent backends delegate to execute().
+        """
+        return self.execute(command, cwd=cwd, timeout=timeout,
+                            stdin_data=stdin_data)
+
     def _timeout_result(self, timeout: int | None) -> dict:
         """Standard return dict when a command times out."""
         return {