Add MEXC exchange support

[mtshit]

Summary

• add direct REST and native protobuf websocket support for MEXC spot market data, public streams, spot trading, and account balances
• bind MEXC capabilities as spot-only with direct-managed transport and expose MEXC extension keys
• add conversion/error handling coverage for order IDs, market statuses, rate limits, raw REST errors, WS topics, and protobuf payloads

Verification

• cargo fmt --check
• cargo test -p mkt-exchange-mexc
• cargo clippy -p mkt-exchange-mexc --all-targets -- -D warnings
• cargo test -p mkt --no-default-features
• cargo test -p mkt --features mexc

Notes

• MEXC has no official SDK; implementation uses direct REST and websocket/protobuf handling.
• Unsupported unified public stream subscriptions remain explicitly rejected where no MEXC topic is available.