test: isolate remote thread store regression from plugin warmups

[bolinfest]

Follow-up to #19266.

Why

thread_start_with_non_local_thread_store_does_not_create_local_persistence is meant to catch accidental local thread persistence when a non-local thread store is configured. The Windows flake reported in this BuildBuddy invocation showed that the assertion was tripping on an unexpected top-level .tmp entry:

 {
+    ".tmp",
     "config.toml",
     "installation_id",
     "memories",
     "skills",
 }

That .tmp does not appear to come from tempfile::TempDir; it comes from unrelated plugin startup work that can legitimately materialize codex_home/.tmp, including the startup remote plugin sync marker in core/src/plugins/startup_sync.rs and the curated plugin snapshot under .tmp/plugins.

That makes the regression race unrelated background startup tasks instead of validating the thread-store invariant it was added to cover. Rather than weakening the assertion to allow arbitrary .tmp entries, this change isolates the test from plugin warmups so it can stay strict about unexpected local thread persistence artifacts.

What changed

• disable plugins in the generated config used by app-server/tests/suite/v2/remote_thread_store.rs
• keep the existing codex_home assertions unchanged so the test still fails if local session or sqlite persistence is introduced

Verification

• cargo test -p codex-app-server suite::v2::remote_thread_store::thread_start_with_non_local_thread_store_does_not_create_local_persistence -- --exact