test: harden app-server integration tests (#19683)

## Why

Windows Bazel runs in the permissions stack exposed that app-server
integration tests were launching normal plugin startup warmups in every
subprocess. Those warmups can call
`https://chatgpt.com/backend-api/plugins/featured` when a test is not
specifically exercising plugin startup, which adds slow background work,
noisy stderr, and dependence on external network state. The relevant
startup/featured-plugin behavior was introduced across #15042 and
#15264.

A few app-server tests also had long optional waits or unbounded cleanup
paths, making failures expensive to diagnose and contributing to slow
Windows shards. One external-agent config test from #18246 used a
GitHub-style marketplace source, which was enough to exercise the
pending remote-import path but also meant the background completion task
could attempt a real clone.

## What Changed

- Adds explicit `AppServerRuntimeOptions` / `PluginStartupTasks`
plumbing and a hidden debug-only
`--disable-plugin-startup-tasks-for-tests` app-server flag, so
integration tests can suppress startup plugin warmups without adding a
production env-var gate.
- Has the app-server test harness pass that hidden flag by default,
while opting plugin-startup coverage back in for tests that
intentionally exercise startup sync and featured-plugin warmup behavior.
- Lowers normal app-server subprocess logging from `info`/`debug` to
`warn` to avoid multi-megabyte stderr output in Bazel logs.
- Prevents the external-agent config test from attempting a real
marketplace clone by using an invalid non-local source while still
exercising the pending-import completion path.
- Bounds optional filesystem/realtime waits and fake WebSocket
test-server shutdown so failures produce targeted timeouts instead of
hanging a shard.
- Fixes the Unix script-resolution test in `rmcp-client` to exercise
PATH resolution directly and include the actual spawn error in failures.

## Verification

- `cargo check -p codex-app-server`
- `cargo clippy -p codex-app-server --tests -- -D warnings`
- `cargo test -p codex-rmcp-client
program_resolver::tests::test_unix_executes_script_without_extension`
- `cargo test -p codex-app-server --test all
external_agent_config_import_sends_completion_notification_after_pending_plugins_finish
-- --nocapture`
- `cargo test -p codex-app-server --test all
plugin_list_uses_warmed_featured_plugin_ids_cache_on_first_request --
--nocapture`
- Windows Local Bazel passed with this test-hardening bundle before it
was extracted from #19606.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/19683).
* #19395
* #19394
* #19393
* #19392
* #19606
* __->__ #19683

codex-rs/app-server/tests/suite/v2/fs.rs

@@ -33,6 +33,7 @@ use std::process::Command;
 const DEFAULT_READ_TIMEOUT: Duration = Duration::from_secs(60);
 #[cfg(not(any(target_os = "macos", windows)))]
 const DEFAULT_READ_TIMEOUT: Duration = Duration::from_secs(10);
+const OPTIONAL_FS_CHANGE_TIMEOUT: Duration = Duration::from_secs(2);
 
 async fn initialized_mcp(codex_home: &TempDir) -> Result<McpProcess> {
     let mut mcp = McpProcess::new(codex_home.path()).await?;
@@ -832,7 +833,7 @@ async fn maybe_fs_changed_notification(
     mcp: &mut McpProcess,
 ) -> Result<Option<FsChangedNotification>> {
     match timeout(
-        DEFAULT_READ_TIMEOUT,
+        OPTIONAL_FS_CHANGE_TIMEOUT,
         mcp.read_stream_until_notification_message("fs/changed"),
     )
     .await
@@ -845,6 +846,14 @@ async fn maybe_fs_changed_notification(
 fn replace_file_atomically(path: &PathBuf, contents: &str) -> Result<()> {
     let temp_path = path.with_extension("lock");
     std::fs::write(&temp_path, contents)?;
+
+    #[cfg(windows)]
+    match std::fs::remove_file(path) {
+        Ok(()) => {}
+        Err(err) if err.kind() == std::io::ErrorKind::NotFound => {}
+        Err(err) => return Err(err.into()),
+    }
+
     std::fs::rename(temp_path, path)?;
     Ok(())
 }