perf(invoice): optimize index updates in create

[Olasunkanmi975]

##closed #141

Summary

Optimizes the InvoiceContract::create flow by reducing redundant persistent storage writes during index maintenance while preserving existing contract behavior and index correctness.

Changes

• Refactored index update logic to minimize storage set operations.
• Consolidated redundant index maintenance where possible.
• Preserved issuer, buyer, and status index semantics.
• Added tests verifying index correctness after invoice creation.
• Added documentation describing the optimization and its safety.

Testing

Covered:

• Invoice creation.
• Issuer index integrity.
• Buyer index integrity.
• Status index integrity.
• Regression checks for unchanged observable behavior.

Validation

cargo fmt
cargo check --workspace
cargo test

Performance Notes

• Reduced the number of persistent storage writes performed during create.
• Preserved storage layout and external contract behavior.
• No changes to authorization, overflow handling, or public APIs.

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[Olasunkanmi975]

@K1NGD4VID please review and merge

[K1NGD4VID]

Fix Ci Error @Olasunkanmi975