Stabilize plugin MCP fixture tests (#19452)

## Why

Recent `main` CI had repeated flakes in the plugin fixture tests:

- `codex-core::all
suite::plugins::explicit_plugin_mentions_inject_plugin_guidance` failed
in runs
[24909500958](https://github.com/openai/codex/actions/runs/24909500958),
[24908076251](https://github.com/openai/codex/actions/runs/24908076251),
[24906197645](https://github.com/openai/codex/actions/runs/24906197645),
and
[24898949647](https://github.com/openai/codex/actions/runs/24898949647).
- `codex-core::all suite::plugins::plugin_mcp_tools_are_listed` failed
in runs
[24909500958](https://github.com/openai/codex/actions/runs/24909500958),
[24908076251](https://github.com/openai/codex/actions/runs/24908076251),
and
[24898949647](https://github.com/openai/codex/actions/runs/24898949647).

The failures were in the same plugin/MCP fixture family: assertions
expected sample plugin guidance or tool inventory, but the test could
observe the session before the sample MCP server had finished startup.

## Root Cause

`explicit_plugin_mentions_inject_plugin_guidance` submitted the user
turn immediately after constructing the session. MCP startup is
asynchronous, so on a slower or busier CI runner the prompt could be
built before the sample plugin MCP server had reported its tools. That
made the test depend on scheduler timing rather than the fixture being
ready.

`plugin_mcp_tools_are_listed` already needed the same readiness
condition, but its wait logic was local to that test.

## What Changed

- Added a shared `wait_for_sample_mcp_ready` helper for the plugin
fixture tests.
- Wait for `McpStartupComplete` before submitting the explicit plugin
mention turn.
- Reuse the same readiness helper in the MCP tool-listing test.

## Why This Should Be Reliable

The tests now wait for the explicit readiness signal from the sample MCP
server before asserting guidance or tools derived from that server. This
removes the startup race while still exercising the real fixture path,
so the assertions should only run after the plugin inventory is
deterministic.

## Verification

- `cargo test -p codex-core --test all plugins::`
- GitHub CI for this PR is passing.

codex-rs/core/tests/suite/plugins.rs

@@ -153,6 +153,45 @@ async fn build_apps_enabled_plugin_test_codex(
         .codex)
 }
 
+async fn wait_for_sample_mcp_ready(codex: &codex_core::CodexThread) -> Result<()> {
+    let startup_event = wait_for_event_with_timeout(
+        codex,
+        |ev| match ev {
+            EventMsg::McpStartupComplete(summary) => {
+                summary.ready.iter().any(|server| server == "sample")
+                    || summary
+                        .failed
+                        .iter()
+                        .any(|failure| failure.server == "sample")
+                    || summary.cancelled.iter().any(|server| server == "sample")
+            }
+            _ => false,
+        },
+        Duration::from_secs(70),
+    )
+    .await;
+    let EventMsg::McpStartupComplete(startup) = startup_event else {
+        unreachable!("event guard guarantees McpStartupComplete");
+    };
+    if let Some(failure) = startup
+        .failed
+        .iter()
+        .find(|failure| failure.server == "sample")
+    {
+        let error = &failure.error;
+        bail!("plugin MCP server failed to start: {error}");
+    }
+    if startup.cancelled.iter().any(|server| server == "sample") {
+        bail!("plugin MCP server startup was cancelled");
+    }
+    assert!(
+        startup.ready.iter().any(|server| server == "sample"),
+        "expected plugin MCP server to be ready; startup summary: {startup:?}"
+    );
+
+    Ok(())
+}
+
 fn tool_names(body: &serde_json::Value) -> Vec<String> {
     body.get("tools")
         .and_then(serde_json::Value::as_array)
@@ -268,6 +307,7 @@ async fn explicit_plugin_mentions_inject_plugin_guidance() -> Result<()> {
     let codex =
         build_apps_enabled_plugin_test_codex(&server, codex_home, apps_server.chatgpt_base_url)
             .await?;
+    wait_for_sample_mcp_ready(&codex).await?;
 
     codex
         .submit(Op::UserInput {
@@ -416,41 +456,7 @@ async fn plugin_mcp_tools_are_listed() -> Result<()> {
     let rmcp_test_server_bin = stdio_server_bin()?;
     write_plugin_mcp_plugin(codex_home.as_ref(), &rmcp_test_server_bin);
     let codex = build_plugin_test_codex(&server, codex_home).await?;
-
-    let startup_event = wait_for_event_with_timeout(
-        &codex,
-        |ev| match ev {
-            EventMsg::McpStartupComplete(summary) => {
-                summary.ready.iter().any(|server| server == "sample")
-                    || summary
-                        .failed
-                        .iter()
-                        .any(|failure| failure.server == "sample")
-                    || summary.cancelled.iter().any(|server| server == "sample")
-            }
-            _ => false,
-        },
-        Duration::from_secs(70),
-    )
-    .await;
-    let EventMsg::McpStartupComplete(startup) = startup_event else {
-        unreachable!("event guard guarantees McpStartupComplete");
-    };
-    if let Some(failure) = startup
-        .failed
-        .iter()
-        .find(|failure| failure.server == "sample")
-    {
-        let error = &failure.error;
-        bail!("plugin MCP server failed to start: {error}");
-    }
-    if startup.cancelled.iter().any(|server| server == "sample") {
-        bail!("plugin MCP server startup was cancelled");
-    }
-    assert!(
-        startup.ready.iter().any(|server| server == "sample"),
-        "expected plugin MCP server to be ready; startup summary: {startup:?}"
-    );
+    wait_for_sample_mcp_ready(&codex).await?;
 
     codex.submit(Op::ListMcpTools).await?;
     let list_event = wait_for_event_with_timeout(