feat(ctrader-openapi): complete client API with orders, symbols, account, market data

Add rate limiter (50/s + 5/s historical), symbol mapper, order builders,
account queries, market data subscriptions, and high-level CTraderClient
that orchestrates the full connect/auth/symbol-load lifecycle.

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

ctrader-openapi/src/error.rs

@@ -72,6 +72,12 @@ impl From<std::io::Error> for CTraderError {
     }
 }
 
+impl From<prost::DecodeError> for CTraderError {
+    fn from(e: prost::DecodeError) -> Self {
+        Self::ProtocolError(format!("protobuf decode error: {e}"))
+    }
+}
+
 /// Rate-limit bucket categories.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum RateLimitBucket {