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/app-server/src/request_processors/mcp_processor.rs

@@ -183,9 +183,10 @@ impl McpRequestProcessor {
         };
         let resolved_scopes =
             resolve_oauth_scopes(scopes, server.scopes.clone(), discovered_scopes);
+        let oauth_credential_name = server.oauth_credential_name(&name);
 
         let handle = perform_oauth_login_return_url(
-            &name,
+            oauth_credential_name.as_ref(),
             &url,
             mcp_config.mcp_oauth_credentials_store_mode,
             mcp_config.auth_keyring_backend_kind,