add createdAt field to index orders table (backport #3327)

[mergify]

Changelist

[Describe or list the changes made in this PR]

Test Plan

[Describe how this PR was tested (if applicable)]

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

• New Features

  • Orders now include a createdAt timestamp field in API responses, displaying when each order was created.

• Tests

  • Updated test assertions to validate the new createdAt field.

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This is an automatic backport of pull request #3327 done by [Mergify](https://mergify.com).