feat(sql): add x402 and MPP payment lanes - #60

Adds SQL Explorer without a Quicknode account: wallet, fund, query. The account-key path stays.

qn sql clusters and qn sql schema always hit the public catalog. Only qn sql query chooses who pays: an API key, --x402-drawdown, or --mpp-session. Config never turns a payment path on.

qn micropayments (alias pay) is the shared funding noun. qn rpc x402 and qn rpc mpp stay first-class and call the same runners.

# Keyless: generate a wallet, fund it, query.
qn wallet generate --chain eip155:84532
qn micropayments x402 buy-credits \
  --payment-key-file ./key.txt \
  --payment-network eip155:84532 \
  --payment-asset 0x036CbD53842c5426634e7929541eC2318f3dCF7e \
  --max-amount 10000000 --network base-sepolia --yes
qn sql query "SELECT 1" \
  --cluster-id hyperliquid-core-mainnet \
  --x402-drawdown \
  --payment-key-file ./key.txt \
  --payment-network base-sepolia

Channel accounting on the MPP lane

The paid query saves the channel on both the success and error paths, before mapping the error:

// Persist before inspecting: a failed query can still have spent the voucher.
let result = ctx
    .sdk
    .sql
    .query_with_mpp_session(&params, &payment, &mut channel)
    .await;
if let Some(path) = &channels_path {
    let _ = config::save_channel(path, &scope, &channel);
}
let result = result.map_err(map_sql_mpp_error)?;

The SDK advances channel whenever the voucher reached the gateway. Previously the CLI returned on error before saving, so a query that failed after the gateway banked the voucher left the cached cumulative behind the gateway's accepted value; the next query re-signed that stale value and was refused, stranding the channel.

Error matching

Both payment matchers now gate on the status. The body markers were ||-alternatives to the 402 check, so a 500 whose message happened to contain "insufficient" reported as an empty balance:

// Status only: body text on a 500 saying "insufficient" is not an empty balance.
fn is_sql_requires_payment(e: &SdkError) -> bool {
    matches!(e, SdkError::Api { status, .. } if status.as_u16() == 402)
}

Docs

• sql query has no per-request --x402/--mpp flag; context.md no longer says the flag is "rejected" (clap emits a generic unknown-argument error).
• The no-payer sql query is documented as exit 1. A key is one of three ways to pay, so a missing payer is a usage error, not no API key found (exit 4, which stays the default for key-required commands).

Dependency

Pinned to quicknode/sdk@0bd4f3c (quicknode/sdk#60), git-pinned until that crate is published. A release cannot ship from a git rev.

Closes DX-6602

Test plan

• cargo test --test sql --test micropayments --test rpc_payment --test cli_smoke — 86 passed
• cargo test — 462 passed
• cargo clippy --all-targets -- -D warnings
• cargo fmt --check
• CI

New coverage: a failed MPP query persists the receipt cumulative; sql clusters maps a catalog error to exit 2; buy-credits clears the confirmation gate with --yes; insta snapshot for the sql clusters table.