Isolate MCP OAuth credentials by environment (#36310)

## Why

MCP servers running in an executor must not reuse host-owned OAuth credentials,
even when they have the same configured server name and URL.

## What changed

- Derive environment-scoped credential names for executor-owned MCP servers and
  use them consistently for login, logout, authentication, and connection setup.
- Mark executor-owned entries in the file credential store and fail closed when
  loading or saving an entry could cross the host/executor boundary.
- Preserve compatibility with existing local OAuth credentials, including local
  server names that overlap the new reserved prefixes.

## Testing

- Extend the executor MCP integration test to verify that executor requests use
  the executor token, never send the host token, and persist both credentials
  separately.

GitOrigin-RevId: 4fc92d0533b0fe2e0df34f6d47b81e3d20d07807

codex-rs/core/src/mcp_skill_dependencies.rs

@@ -169,8 +169,9 @@ pub(crate) async fn maybe_install_mcp_dependencies(
             oauth_config.discovered_scopes.clone(),
         );
         let oauth_client_id = server_config.oauth_client_id();
+        let oauth_credential_name = server_config.oauth_credential_name(&name);
         let first_attempt = perform_oauth_login(
-            &name,
+            oauth_credential_name.as_ref(),
             &oauth_config.url,
             config.mcp_oauth_credentials_store_mode,
             config.auth_keyring_backend_kind(),
@@ -188,7 +189,7 @@ pub(crate) async fn maybe_install_mcp_dependencies(
         if let Err(err) = first_attempt {
             if should_retry_without_scopes(&resolved_scopes, &err) {
                 if let Err(err) = perform_oauth_login(
-                    &name,
+                    oauth_credential_name.as_ref(),
                     &oauth_config.url,
                     config.mcp_oauth_credentials_store_mode,
                     config.auth_keyring_backend_kind(),