fix: fix collision handling in comlink username task (backport #3293)

[mergify]

Changelist

• Remove transactions from subaccount username generator task to prevent username collisions breaking non-collided inserts
• Fix small test case for subaccount usernames(we now seed 2 subaccount usernames already)

Test Plan

This was tested locally using the test suite

Author/Reviewer Checklist

• If this PR has changes that result in a different app state given the same prior state and transaction list, manually add the state-breaking label.
• If the PR has breaking postgres changes to the indexer add the indexer-postgres-breaking label.
• If this PR isn't state-breaking but has changes that modify behavior in PrepareProposal or ProcessProposal, manually add the label proposal-breaking.
• If this PR is one of many that implement a specific feature, manually label them all feature:[feature-name].
• If you wish to for mergify-bot to automatically create a PR to backport your change to a release branch, manually add the label backport/[branch-name].
• Manually add any of the following labels: refactor, chore, bug.

Summary by CodeRabbit

• Tests

  • Added tests for fallback username selection when conflicts occur, mixed-batch scenarios, and switched username lookups to read-replica assertions.

• Refactor

  • Simplified subaccount username generation flow with clearer success/failure handling, improved retry/fallback behavior, adjusted transaction handling, and updated logging.

• Chores

  • Added an insert-with-conflict-feedback operation to detect whether a username was actually created.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

---

This is an automatic backport of pull request #3293 done by [Mergify](https://mergify.com).

[coderabbitai]

Important

Review skipped

Bot user detected.

To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[lukabuz]

https://github.com/Mergifyio backport release/indexer/v9.x

[mergify]

backport release/indexer/v9.x

✅ Backports have been created

Details

• Backport to branch release/indexer/v9.x not needed, change already in branch release/indexer/v9.x