fix(e2e): make fee_settings test resilient to fee decay

[AztecBot]

Summary

Fixes CI failure in e2e_fees/fee_settings.test.ts on merge-train/fairies.

The test captured stableMinFees early, then prepared two txs (takes time for proving). During this window, L2 fees naturally decayed due to low utilization. A single 20% bump was insufficient to bring fees back above the originally captured level.

Fix: bumpL2Fees() now accepts an optional minTarget and loops — bumping proving cost by 20% each iteration — until fees exceed the target. Each 20% increment is small enough to stay under the 1.5x padding threshold.

Full analysis: https://gist.github.com/AztecBot/a0d2cf5bccf8044345f37473510d21c6

ClaudeBox log: https://claudebox.work/s/395c6f5ae76e825e?run=1

[AztecBot]

Automatically closing this stale claudebox draft PR (no updates for 5+ days). Re-open if still needed.