Reject report_revenue override_existing=true on nonexistent periods instead of treating as initial

[thlpkee20-wq]

Description

When report_revenue is called with override_existing=true for a period_id that has no prior persisted report, the override flag is effectively ignored and the call falls through to initial-report handling. This conflates "correct an existing period" with "create a new period" and can mask off-chain bugs. Make override_existing=true require an existing report, returning a typed error otherwise.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: src/lib.rs (report_revenue match on reports.get(period_id))
• The override path must still emit rev_ovrd and apply net delta to total_revenue without incrementing report_count

Suggested execution

• Fork the repo and create a branch
• git checkout -b feat/override-requires-existing-period
• Implement changes
  • In the None arm, branch on override_existing and return a clear error (e.g. InvalidPeriodId or new variant)
  • Preserve existing initial-report behavior when override_existing=false
  • Update README report_revenue semantics
• Validate security and correctness assumptions

Test and commit

• Run tests
  • cargo test
• Cover edge cases
  • override on new period, override on existing, initial then override, event_only mode
• Include test output and security notes

Example commit message

feat: require existing period for report_revenue override_existing

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours

[Zainab5970]

@Zainab5970 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

please can i work on the issue

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This issue has been completed by @Zainab5970 as part of the Stellar Wave Program's 5th Wave 🥳

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