Refactor GPU environment verification and SSH execution for improved flexibility

- Removed the direct injection of environment variables into the script, preventing issues with `from __future__` imports.
- Updated the `run_python` function to accept an `env` parameter for passing runtime environment variables, enhancing script execution control.
- Adjusted tests to verify the new behavior, ensuring that environment variables are correctly set without modifying the script content.

src/gpu_rent/ssh_ops.py

@@ -352,8 +352,13 @@ def run_python(
     remote_path: str,
     timeout: int = 1800,
     log: Callable[[str], None] | None = None,
+    env: dict[str, str] | None = None,
 ) -> str:
-    """Upload a Python script and run it as SSH user (not root)."""
+    """Upload a Python script and run it as SSH user (not root).
+
+    Do not prepend Python code to ``script`` (breaks ``from __future__``).
+    Pass runtime flags via ``env`` instead.
+    """
     client = _connect(cfg, host)
     chunks: list[str] = []
     try:
@@ -362,7 +367,16 @@ def run_python(
             fh.write(script)
         sftp.chmod(remote_path, 0o755)
         sftp.close()
-        command = f"python3 {shlex.quote(remote_path)}"
+        exports = ""
+        if env:
+            parts = [
+                f"{key}={shlex.quote(str(val))}"
+                for key, val in env.items()
+                if key
+            ]
+            if parts:
+                exports = " ".join(parts) + " "
+        command = f"{exports}python3 {shlex.quote(remote_path)}"
         _stdin, stdout, stderr = client.exec_command(command, timeout=timeout, get_pty=True)
         _stream_pty_output(stdout, log=log, chunks=chunks)
         code = stdout.channel.recv_exit_status()