fix: fix serde_as annotation and verify with test (#3170)

I didn't do https://github.com/openai/codex/pull/3163 correctly the
first time: now verified with a test.

codex-rs/protocol/Cargo.toml

@@ -17,7 +17,7 @@ mime_guess = "2.0.5"
 serde = { version = "1", features = ["derive"] }
 serde_bytes = "0.11"
 serde_json = "1"
-serde_with = "3.14.0"
+serde_with = { version = "3.14.0", features = ["macros", "base64"] }
 strum = "0.27.2"
 strum_macros = "0.27.2"
 tracing = "0.1.41"