refactor: make code-mode crate wasm-safe

Gate the native V8 dependency out of wasm builds and provide a wasm-specific default CodeModeService that preserves the shared request/response/runtime trait surface while returning clear errors unless a browser runtime is injected. Keep the public code-mode types available on wasm and mark the V8-backed runtime internals as native-only so codex-code-mode now checks cleanly for wasm32-unknown-unknown.

codex-rs/code-mode/src/lib.rs

@@ -1,6 +1,7 @@
 mod description;
 mod response;
 mod runtime;
+#[cfg_attr(target_arch = "wasm32", path = "service_wasm.rs")]
 mod service;
 
 pub use description::CODE_MODE_PRAGMA_PREFIX;