apollo_mempool: fix gap comment and add stale delayed declare test

[ArniStarkware]

Note

Low Risk
Low risk: only a comment tweak in update_accounts_with_gap plus a new unit test; no runtime behavior changes.

Overview
Adds a new unit test (stale_delayed_declare_does_not_suppress_gap_detection) to ensure a stale delayed Declare (overtaken by a committed nonce) does not incorrectly clear accounts_with_gap in multi-sequencer scenarios.

Clarifies the comment in Mempool::update_accounts_with_gap to state that a delayed transaction at the current account nonce means the next nonce is available, so no gap should be reported.

^{Written by Cursor Bugbot for commit ce9150c. This will update automatically on new commits. Configure here.}

[reviewable-StarkWare]

This change is 

[ArniStarkware]

• apollo_mempool: fix gap comment and add stale delayed declare test #13624 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

[ArniStarkware]

@ArniStarkware reviewed 2 files.
Reviewable status: 2 of 4 files reviewed, all discussions resolved (waiting on AvivYossef-starkware).