feat(sql): add catalog list and paid query sessions

[johnpmitsch]

Adds SQL Explorer methods for a public catalog and two paid query lanes.

list_clusters reads GET /sql/rest/v1/clusters on a keyless client. query_with_session sends a SIWX Bearer token and does not sign a per-request payment. query_with_mpp_session posts the SQL body to the MPP session route and advances the voucher by the SQL challenge amount, not per_call.

Channel accounting

query_with_mpp_session takes &mut ChannelState and advances cumulative_spent whenever the voucher reached the gateway — a non-2xx body and a lost response included. Only a connect failure, which never puts the voucher on the wire, leaves the channel untouched.

The gateway refuses a re-signed stale cumulative, so a channel left behind the gateway cannot be used again, while one slightly ahead still works. Callers persist on every outcome, not just after a 200:

let result = qn
    .sql
    .query_with_mpp_session(&params, &payment, &mut channel)
    .await;
save_channel(&channel)?; // persist first — the voucher may have settled
let result = result?;

Previously the payment-receipt header was parsed and then discarded on any non-2xx, so a gateway that banked a voucher and then failed the query body stranded the channel.

Test plan

• cargo test -p quicknode-sdk --features rust,payments,payments-tempo --lib sql — 13 passed
• cargo test -p quicknode-sdk --features rust,payments,payments-tempo --lib — 331 passed
• cargo fmt --check
• CI

New coverage: a 500 carrying a receipt advances the channel to the receipt value; a 402 with no receipt advances to the signed value; a failed probe leaves the channel unchanged.

Notes

cargo clippy --all-targets reports three pre-existing E0308 errors in examples/streams.rs and examples/webhooks_e2e.rs (stream compression, webhook templates). Confirmed present with this branch's changes stashed; untouched here.

Bindings parity for the new SQL surface (Python/Node/Ruby) is not in this PR.