chore(prover-client): demote empty addTxs log from warn to verbose (#24593)

`CheckpointSubTreeOrchestrator.addTxs` logs `Provided no txs to addTxs.`
at **warn** level, but this path is expected control flow, not an
anomaly: `checkpoint-prover.ts` calls `this.subTree.addTxs(processed)`
unconditionally for every block in the checkpoint, so every empty block
hits this early return (the empty block is then finalized via
`setBlockCompleted`). On low-traffic networks this fires constantly —
~100 times in 4h on staging-internal — polluting warn-level log reviews
with noise.

Demote it to `verbose`. No behavior change; no tests assert on this
message.

Replaces #24591 and #24592 (closed), per request to land this as a
single PR via the spartan-v5 merge train.

Spotted during a staging-internal log review requested by @spalladino in
Slack.

---
*Created by
[claudebox](https://claudebox.work/v2/sessions/ac050bae673edac4) ·
group: `slackbot`*

Author: AztecBot

yarn-project/prover-client/src/orchestrator/checkpoint-sub-tree-orchestrator.ts

@@ -323,7 +323,7 @@ export class CheckpointSubTreeOrchestrator extends ProvingScheduler {
     if (!txs.length) {
       // Empty block: setBlockCompleted handles this without addTxs being called. Bail to
       // avoid the throw below (we cannot find the blockNumber without any txs).
-      this.logger.warn(`Provided no txs to addTxs.`);
+      this.logger.verbose(`Provided no txs to addTxs.`);
       return;
     }