feat(nsjail): optional disk-backed /tmp via instance setting (#9272)

* feat(nsjail): optional disk-backed /tmp via instance setting

* test(nsjail): unit-test tmp mount resolver and narrow visibility

* refactor(nsjail): switch tmp backing to select + conditional UI

* ui(nsjail): make tmpfs the visible default in /tmp backing select

* fix(nsjail): refuse preexisting jail_tmp to block symlink escape

* fix(nsjail): allow jail_tmp reuse on sequential nsjail calls

Codex flagged that python/ruby/rust executors invoke nsjail twice per
job_dir (install then run). The previous resolver treated any preexisting
jail_tmp as hostile and silently fell back to tmpfs on the second call,
so disk-backed mode never reached the main script run for those langs.

Use symlink_metadata().is_dir() to distinguish a real directory left by
an earlier call in the same job_dir (safe to reuse) from a symlink or
other entity (still refused, as the codebase-tar escape requires).

Also loosen the frontend visibility predicate: only hide nsjail settings
when job_isolation is explicitly 'none' or 'unshare', so deployments
that enable nsjail via DISABLE_NSJAIL=false with no DB setting can
still see the controls.

backend/windmill-worker/src/python_executor.rs

@@ -146,7 +146,7 @@ use windmill_object_store::OBJECT_STORE_SETTINGS;
 use crate::{
     common::{
         build_command_with_isolation, create_args_and_out_file, get_reserved_variables, read_file,
-        read_result, resolve_nsjail_timeout, resolve_nsjail_tmpfs_size_bytes, start_child_process,
+        read_result, resolve_nsjail_timeout, resolve_nsjail_tmp_mount_block, start_child_process,
         OccupancyMetrics, StreamNotifier, DEV_CONF_NSJAIL,
     },
     get_proxy_envs_for_lang,
@@ -1028,8 +1028,8 @@ mount {{
                 .replace("{TRACING_PROXY_CA_CERT_PATH}", &*TRACING_PROXY_CA_CERT_PATH)
                 .replace("#{DEV}", DEV_CONF_NSJAIL)
                 .replace(
-                    "{NSJAIL_TMPFS_SIZE}",
-                    &resolve_nsjail_tmpfs_size_bytes().await,
+                    "{TMP_MOUNT_BLOCK}",
+                    &resolve_nsjail_tmp_mount_block(job_dir).await,
                 )
                 .replace("{TIMEOUT}", &nsjail_timeout),
         )?;
@@ -2056,8 +2056,8 @@ async fn spawn_uv_install(
                 .replace("{TRACING_PROXY_CA_CERT_PATH}", &*TRACING_PROXY_CA_CERT_PATH)
                 .replace("#{DEV}", DEV_CONF_NSJAIL)
                 .replace(
-                    "{NSJAIL_TMPFS_SIZE}",
-                    &resolve_nsjail_tmpfs_size_bytes().await,
+                    "{TMP_MOUNT_BLOCK}",
+                    &resolve_nsjail_tmp_mount_block(job_dir).await,
                 )
                 .as_str(),
         )?;