M3 #28: Rust SDK: HTTP client for REST API

[joaquinbejar]

Summary

Implements the HTTP client that wraps the REST API endpoints, providing a type-safe interface for fetching market data, order book snapshots, and user data.

Changes

New Module: sdk/src/client/

File	Description
mod.rs	Module exports
config.rs	Client configuration
error.rs	Client-specific errors
http.rs	HTTP client implementation

Client Configuration

• Base URL: Configurable API endpoint
• Timeout: Request timeout (default: 30s)
• Max Retries: Automatic retry count (default: 3)
• API Key: Optional authentication
• User Agent: Customizable user agent

Market Data Methods

• get_markets() → Vec<Market>
• get_market(address) → Market
• get_orderbook(market, depth) → OrderBook
• get_trades(market, limit, before) → Vec<Trade>

User Data Methods

• get_orders(owner, market) → Vec<Order>
• get_user_trades(owner, market) → Vec<Trade>
• get_balances(owner) → Vec<Balance>

Error Handling

• ClientError::Request — HTTP request failures
• ClientError::Deserialization — JSON parsing errors
• ClientError::Api — API error responses
• ClientError::RateLimited — 429 with retry-after
• ClientError::NotFound — 404 responses
• ClientError::Unauthorized — 401 responses
• ClientError::Timeout — Request timeouts

Dependencies

• Add reqwest v0.12 with json and rustls-tls features

Technical Decisions

• reqwest: Industry-standard async HTTP client
• Automatic retry: Exponential backoff for transient failures
• Rate limiting: Respects Retry-After header on 429
• Connection pooling: Reuses connections via reqwest
• Typed responses: Deserializes into SDK types

Testing

• 16 unit tests for client configuration and creation
• Configuration validation tests
• Builder pattern tests

Checklist

• Code follows .internalDoc/09-rust-guidelines.md
• Documentation added for all public items
• No warnings from cargo clippy
• cargo fmt passes
• All tests pass

Closes #28