feat(ctrader-openapi): length-delimited protobuf codec

CTraderCodec implements tokio_util Decoder+Encoder with 4-byte BE length
prefix framing. Handles partial frames, multiple frames in buffer, and
rejects oversized frames (16 MiB max). 5 unit tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ctrader-openapi/src/error.rs

@@ -60,6 +60,16 @@ pub enum CTraderError {
     /// Symbol not found in the cached symbol list.
     #[error("unknown symbol: {0}")]
     UnknownSymbol(String),
+
+    /// I/O error (required by `tokio_util::codec`).
+    #[error("io error: {0}")]
+    Io(String),
+}
+
+impl From<std::io::Error> for CTraderError {
+    fn from(e: std::io::Error) -> Self {
+        Self::Io(e.to_string())
+    }
 }
 
 /// Rate-limit bucket categories.