fix: ensure accept_elicitation_for_prompt_rule() test passes locally (#7832)

When I originally introduced `accept_elicitation_for_prompt_rule()` in
https://github.com/openai/codex/pull/7617, it worked for me locally
because I had run `codex-rs/exec-server/tests/suite/bash` once myself,
which had the side-effect of installing the corresponding DotSlash
artifact.

In CI, I added explicit logic to do this as part of
`.github/workflows/rust-ci.yml`, which meant the test also passed in CI,
but this logic should have been done as part of the test so that it
would work locally for devs who had not installed the DotSlash artifact
for `codex-rs/exec-server/tests/suite/bash` before. This PR updates the
test to do this (and deletes the setup logic from `rust-ci.yml`),
creating a new `DOTSLASH_CACHE` in a temp directory so that this is
handled independently for each test.

While here, also added a check to ensure that the `codex` binary has
been built prior to running the test, as we have to ensure it is
symlinked as `codex-linux-sandbox` on Linux in order for the integration
test to work on that platform.

codex-rs/exec-server/tests/suite/accept_elicitation.rs

@@ -1,9 +1,12 @@
 #![allow(clippy::unwrap_used, clippy::expect_used)]
 use std::borrow::Cow;
+use std::path::PathBuf;
 use std::sync::Arc;
 use std::sync::Mutex;
 
+use anyhow::Context;
 use anyhow::Result;
+use anyhow::ensure;
 use codex_exec_server::ExecResult;
 use exec_server_test_support::InteractiveClient;
 use exec_server_test_support::create_transport;
@@ -17,6 +20,7 @@ use rmcp::model::CallToolResult;
 use rmcp::model::CreateElicitationRequestParam;
 use rmcp::model::object;
 use serde_json::json;
+use std::os::unix::fs::PermissionsExt;
 use std::os::unix::fs::symlink;
 use tempfile::TempDir;
 
@@ -42,7 +46,9 @@ prefix_rule(
         codex_home.as_ref(),
     )
     .await?;
-    let transport = create_transport(codex_home.as_ref())?;
+    let dotslash_cache_temp_dir = TempDir::new()?;
+    let dotslash_cache = dotslash_cache_temp_dir.path();
+    let transport = create_transport(codex_home.as_ref(), dotslash_cache).await?;
 
     // Create an MCP client that approves expected elicitation messages.
     let project_root = TempDir::new()?;
@@ -68,11 +74,8 @@ prefix_rule(
     let linux_sandbox_exe_folder = TempDir::new()?;
     let codex_linux_sandbox_exe = if cfg!(target_os = "linux") {
         let codex_linux_sandbox_exe = linux_sandbox_exe_folder.path().join("codex-linux-sandbox");
-        let codex_cli = assert_cmd::Command::cargo_bin("codex")?
-            .get_program()
-            .to_os_string();
-        let codex_cli_path = std::path::PathBuf::from(codex_cli);
-        symlink(&codex_cli_path, &codex_linux_sandbox_exe)?;
+        let codex_cli = ensure_codex_cli()?;
+        symlink(&codex_cli, &codex_linux_sandbox_exe)?;
         Some(codex_linux_sandbox_exe)
     } else {
         None
@@ -129,3 +132,32 @@ prefix_rule(
 
     Ok(())
 }
+
+fn ensure_codex_cli() -> Result<PathBuf> {
+    let codex_cli = PathBuf::from(
+        assert_cmd::Command::cargo_bin("codex")?
+            .get_program()
+            .to_os_string(),
+    );
+
+    let metadata = codex_cli.metadata().with_context(|| {
+        format!(
+            "failed to read metadata for codex binary at {}",
+            codex_cli.display()
+        )
+    })?;
+    ensure!(
+        metadata.is_file(),
+        "expected codex binary at {} to be a file; run `cargo build -p codex-cli --bin codex` before this test",
+        codex_cli.display()
+    );
+
+    let mode = metadata.permissions().mode();
+    ensure!(
+        mode & 0o111 != 0,
+        "codex binary at {} is not executable (mode {mode:o}); run `cargo build -p codex-cli --bin codex` before this test",
+        codex_cli.display()
+    );
+
+    Ok(codex_cli)
+}