Reuse MCP runtimes when selected availability changes nothing (#30148)

## Why

MCP runtime reuse was keyed by every ready selected-capability
environment, even when an environment contributed no MCP servers or
connectors.

For example:

1. a global stdio MCP is running;
2. a selected remote environment contains only a skill;
3. that environment becomes ready;
4. the MCP and connector projection stays exactly the same;
5. Codex nevertheless rebuilds the MCP manager and restarts the global
stdio process.

That restart can interrupt active calls and discard process-local state
even though nothing about MCP changed.

## What changes

When selected-environment availability changes, Codex now resolves the
candidate MCP and connector projection before deciding whether to
replace the runtime:

- if the winning MCP servers or their ownership change, rebuild as
before;
- if the selected connector snapshot changes, rebuild as before;
- if an enabled MCP is explicitly bound to an environment whose
availability changed, rebuild as before;
- otherwise, keep the exact live manager and processes, and update only
the availability input remembered by the snapshot.

```text
ready selected environments:  [] -> [skills-env]
resolved MCP servers:          {global_probe} -> {global_probe}
resolved connectors:           {} -> {}
result:                         reuse manager; keep the same process
```

The comparison uses the resolved winning servers and their sources, so
plugin/config ownership remains part of the runtime identity.

## Existing stack coverage

The integration PR directly below this one already covers both rebuild
boundaries: a selected MCP becomes callable and a selected connector
tool becomes model-visible when their environment becomes available. It
also verifies that an unchanged selected MCP runtime keeps its process.

This PR does not add another remote-attachment integration scenario for
the no-change optimization. `environment/add` returns before readiness,
and app-server does not currently expose a deterministic readiness
signal for an environment that contributes only skills. Keeping a
fixed-delay test would add flake risk; adding a new readiness API would
be outside this fix.

## Scope and assumptions

- This does not change skill discovery, World State rendering, or plugin
metadata caching.
- This does not add file watching or hot reload behavior.
- This does not change disconnect/reconnect handling.
- Selected environment IDs and their capability contents retain the
stack's existing stability assumption.
- Delayed `required = true` executor MCP behavior remains out of scope.

codex-rs/codex-mcp/src/catalog.rs

@@ -371,6 +371,11 @@ impl ResolvedMcpCatalog {
             .collect()
     }
 
+    /// Returns whether both catalogs resolve to the same winning servers and sources.
+    pub fn has_same_servers(&self, other: &Self) -> bool {
+        self.servers == other.servers
+    }
+
     /// Replaces the resolved server set while preserving known server sources.
     ///
     /// Names not present in the existing catalog are treated as config-owned.

codex-rs/core/src/session/mcp.rs

@@ -133,6 +133,36 @@ impl Session {
                 &available_environment_ids,
             )
             .await;
+        let changed_environment_is_used_by_mcp = mcp_config
+            .mcp_server_catalog
+            .configured_servers()
+            .values()
+            .any(|server| {
+                let was_available = current
+                    .available_environment_ids()
+                    .contains(&server.environment_id);
+                let is_available = available_environment_ids.contains(&server.environment_id);
+                server.enabled && was_available != is_available
+            });
+        if !changed_environment_is_used_by_mcp
+            && current
+                .config()
+                .mcp_server_catalog
+                .has_same_servers(&mcp_config.mcp_server_catalog)
+            && current.config().connector_snapshot == mcp_config.connector_snapshot
+        {
+            // Availability is only an input to the MCP projection. When that input changes but
+            // the projected servers and connectors do not, advance the input key without
+            // replacing the live manager and restarting its processes.
+            let runtime = Arc::new(McpRuntimeSnapshot::new(
+                Arc::new(current.config().clone()),
+                current.manager_arc(),
+                current.runtime_context().clone(),
+                available_environment_ids,
+            ));
+            self.services.mcp_runtime.store(Some(Arc::clone(&runtime)));
+            return runtime;
+        }
         self.refresh_mcp_servers_inner(
             turn_context,
             mcp_config,