fix: use a more efficient wire format for ExecCommandOutputDeltaEvent.chunk (#3163)

When serializing to JSON, the existing solution created an enormous
array of ints, which is far more bytes on the wire than a base64-encoded
string would be.

codex-rs/protocol/Cargo.toml

@@ -17,6 +17,7 @@ mime_guess = "2.0.5"
 serde = { version = "1", features = ["derive"] }
 serde_bytes = "0.11"
 serde_json = "1"
+serde_with = "3.14.0"
 strum = "0.27.2"
 strum_macros = "0.27.2"
 tracing = "0.1.41"