Add 28 unit tests for mechanisms/svm/exact/v1/client.py — the only file in mechanisms/svm/exact/v1/ without a dedicated test module. Covers constructor (scheme, signer reference, default/custom RPC URL, empty client cache), _get_client (CAIP-2 devnet/mainnet → default RPC URL, V1 legacy solana-devnet/solana normalization, custom RPC URL priority, per-network caching across legacy + CAIP-2 inputs, unsupported CAIP-2 + unsupported legacy network errors surfaced via normalize_network), and V1 create_payment_payload specifics (missing feePayer in extra, extra=None, mint not found, unknown token program owner, oversized memo, V1 reads max_amount_required (different amounts → different payload bytes), Token-2022 mint owner accepted, default random memo divergence across calls, custom memo round-trips into transaction bytes, non-string memo falls back to random nonce, boundary MAX_MEMO_BYTES accepted).