feat(sdk): add typed error results

[SebastianBoehler]

Summary

• add public SdkError, SdkErrorCode, and Result<T> types for opt-in structured failures
• add result-returning variants for key order creation, posting, cancellation, and order query methods while keeping existing convenience APIs
• classify transport, auth, rate limit, API response, parse, signing, and invalid argument failures with endpoint/status/body excerpt/retry metadata
• capture response headers in HttpResponse so request ids can be surfaced when present
• document the structured error path in the README

Validation

• cmake -S . -B /tmp/polymarket-cpp-client-verify -DPOLYMARKET_CLIENT_BUILD_EXAMPLES=ON -DPOLYMARKET_CLIENT_BUILD_TESTS=ON
• cmake --build /tmp/polymarket-cpp-client-verify -- -j4
• ctest --test-dir /tmp/polymarket-cpp-client-verify --output-on-failure
• git diff --check