[codex] Support flattened deferred MCP tool calls (#17556)

## Summary
- register flattened handler aliases for deferred MCP tools
- cover the node_repl-shaped deferred MCP call path in tool registry
tests

## Root Cause
Deferred MCP tools were registered only under their namespaced handler
key, e.g. `mcp__node_repl__:js`. If the model/bridge emitted the
flattened qualified name `mcp__node_repl__js`, core parsed it as an MCP
payload but dispatch looked up the flattened handler key and returned
`unsupported call` before reaching the MCP handler.

## Validation
- `just fmt`
- `cargo test -p codex-tools
search_tool_registers_deferred_mcp_flattened_handlers`
- `cargo test -p codex-core
search_tool_registers_namespaced_mcp_tool_aliases`
- `git diff --check`

codex-rs/core/src/tools/spec.rs

@@ -263,6 +263,15 @@ pub(crate) fn build_specs_with_discoverable_tools(
             }
         }
     }
+    if let Some(deferred_mcp_tools) = deferred_mcp_tools.as_ref() {
+        for (name, _) in deferred_mcp_tools.iter().filter(|(name, _)| {
+            !mcp_tools
+                .as_ref()
+                .is_some_and(|tools| tools.contains_key(*name))
+        }) {
+            builder.register_handler(name.clone(), mcp_handler.clone());
+        }
+    }
     builder
 }
 

codex-rs/core/src/tools/spec_tests.rs

@@ -909,6 +909,8 @@ fn search_tool_registers_namespaced_mcp_tool_aliases() {
     assert!(registry.has_handler(&ToolName::plain(TOOL_SEARCH_TOOL_NAME)));
     assert!(registry.has_handler(&app_alias));
     assert!(registry.has_handler(&mcp_alias));
+    assert!(registry.has_handler(&ToolName::plain("mcp__codex_apps__calendar_create_event")));
+    assert!(registry.has_handler(&ToolName::plain("mcp__rmcp__echo")));
 }
 
 #[test]