feat(sdk): classify POLY_1271 setup errors

[SebastianBoehler]

Summary

• add a dedicated SdkErrorCode::DepositWalletSetup classification for the known POLY_1271 signer/API-key rejection
• replace the raw CLOB error with guidance to check the deployed funder wallet, approvals, balance, and balance sync
• document the classification in the CLOB V2 migration notes

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