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/Cargo.lock

@@ -923,6 +923,7 @@ dependencies = [
  "serde",
  "serde_bytes",
  "serde_json",
+ "serde_with",
  "strum 0.27.2",
  "strum_macros 0.27.2",
  "tracing",