[FEATURE] Add nightly testnet integration test run

[ritik4ever]

Category: devops / testing
Project area: .github/workflows/nightly.yml (new)

Implementation:

• Scheduled CI (02:00 UTC) runs contract integration tests against Stellar testnet.
• Failure creates GitHub issue with test output.
• Manual trigger via workflow_dispatch.

Acceptance Criteria:

• Nightly job runs on schedule.
• Failure auto-creates issue.
• Manual trigger available.

[Hollujay]

@Hollujay has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I would like to work on this issue because I will get it done.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Hollujay to this issue.