Add opt-in protocol discovery for local stdio MCP servers (#57210)

## Why

Stdio servers without `CODEX_MCP_PROTOCOL_VERSION` use the legacy handshake, even when they support MCP `2026-07-28`.

## What changed

- Add the disabled-by-default `mcp_connection_sharing` feature to enable discovery for eligible local stdio servers.
- Recover from failed or late discovery with a fresh handshake when needed, waiting for the previous helper to release resources before restarting.
- Retain a working legacy connection when an upgrade attempt fails for the same account, except on authentication errors.
- Keep tool catalogs separate by protocol and invalidate legacy catalogs when discovery negotiates the modern protocol.

## Testing

Add regression coverage for failed and late discovery, legacy fallback, protocol-specific catalog reuse, message limits, and Windows descendant cleanup.

GitOrigin-RevId: 36c675ef3b81d32fccf06ccbddda2312cfe602c9

codex-rs/core/config.schema.json

@@ -1143,6 +1143,9 @@
             "mcp_2026_07_28": {
               "type": "boolean"
             },
+            "mcp_connection_sharing": {
+              "type": "boolean"
+            },
             "mcp_oauth_refresh_coordination": {
               "type": "boolean"
             },
@@ -7751,6 +7754,9 @@
         "mcp_2026_07_28": {
           "type": "boolean"
         },
+        "mcp_connection_sharing": {
+          "type": "boolean"
+        },
         "mcp_oauth_refresh_coordination": {
           "type": "boolean"
         },

codex-rs/core/src/config/managed_features.rs

@@ -124,6 +124,7 @@ impl ManagedFeatures {
         let mut values = self.get().clone();
         for feature in [
             Feature::EnableMcpApps,
+            Feature::McpConnectionSharing,
             Feature::SecretAuthStorage,
             Feature::McpOAuthRefreshCoordination,
         ] {

codex-rs/core/src/config/mod.rs

@@ -1957,6 +1957,7 @@ impl Config {
                 Vec::new()
             },
             protocol_mode: self.mcp_protocol_mode(),
+            discover_local_stdio: self.features.enabled(Feature::McpConnectionSharing),
             host_owned_apps_protocol_mode: if self.features.enabled(Feature::CodexAppsMcp20260728) {
                 McpProtocolMode::V20260728
             } else {

codex-rs/core/src/config/runtime_refresh.rs

@@ -67,6 +67,7 @@ impl Config {
         // These operational controls accept host defaults beneath explicit layer values.
         // Enterprise opt-in always comes from trusted layers instead.
         for feature in [
+            Feature::McpConnectionSharing,
             Feature::SecretAuthStorage,
             Feature::McpOAuthRefreshCoordination,
         ] {

codex-rs/core/tests/common/process.rs

@@ -40,7 +40,8 @@ async fn wait_for_process_exit_inner(pid: String) -> anyhow::Result<()> {
 
 pub async fn wait_for_process_exit(pid: &str) -> anyhow::Result<()> {
     let pid = pid.to_string();
-    tokio::time::timeout(Duration::from_secs(2), wait_for_process_exit_inner(pid))
+    // A stdio process group can keep its leader's PID for the two-second shutdown grace.
+    tokio::time::timeout(Duration::from_secs(5), wait_for_process_exit_inner(pid))
         .await
         .context("timed out waiting for process to exit")??;
 

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

@@ -19,6 +19,7 @@ use codex_extension_api::ExtensionRegistryBuilder;
 use codex_extension_api::McpServerContribution;
 use codex_extension_api::McpServerContributionContext;
 use codex_extension_api::McpServerContributor;
+use codex_features::Feature;
 use codex_mcp::CODEX_APPS_MCP_SERVER_NAME;
 use codex_protocol::mcp::McpServerConnectionStatus;
 use codex_protocol::models::PermissionProfile;
@@ -700,6 +701,11 @@ async fn cached_mcp_startup_is_eager_for_root_and_lazy_for_subagents() -> anyhow
         /*sandbox*/ None,
     )
     .await?;
+    // The subagent can reuse the parent's legacy catalog even when discovery is enabled.
+    let mut subagent_config = fixture.config.clone();
+    subagent_config
+        .features
+        .enable(Feature::McpConnectionSharing)?;
     let NewThread {
         thread: second_thread,
         ..
@@ -713,7 +719,7 @@ async fn cached_mcp_startup_is_eager_for_root_and_lazy_for_subagents() -> anyhow
                 agent_nickname: None,
                 agent_role: None,
             })),
-            ..StartThreadOptions::new(fixture.config.clone())
+            ..StartThreadOptions::new(subagent_config)
         })
         .await?;
     second_thread.submit(Op::Interrupt).await?;