Revenue pool: prove `receive_payment` cannot move tokens (docs + tests)

[greatest0fallt1me]

Description

Add tests demonstrating receive_payment does not invoke token transfers; document the threat-model note for ops.

Requirements and context

• Prevents confusion with real settlement.
• Align with SECURITY.md external call checklist.

Suggested execution

• Fork the repo and create a branch: git checkout -b test/revenue-pool-receive-payment-security
• Implement changes in the Callora-Contracts Soroban workspace (contracts/*/src/).
• Run cargo fmt, cargo clippy --all-targets --all-features -- -D warnings, and cargo test from the workspace root.
• For WASM release builds: cargo build --target wasm32-unknown-unknown --release -p callora-revenue-pool (adjust -p if the crate name differs).

Deliverables

• test.rs + rustdoc.

Test and commit

• Run tests and ./scripts/coverage.sh (or cargo tarpaulin per tarpaulin.toml).
• Cover edge cases; include summarized test output and brief security notes in the PR description.

Example commit message

test(revenue-pool): receive_payment does not transfer tokens

Guidelines

• Minimum 95% line coverage for touched crates (workspace policy).
• Clear documentation (Rust /// on public items where applicable; repo markdown as needed).
• Timeframe: 96 hours from assignment unless agreed otherwise.

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Congratulations, @icodeBisola! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @icodeBisola: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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