Settlement: standardize unauthorized panic strings + tests

[greatest0fallt1me]

Description

Align unauthorized: caller must be vault or admin messaging across entrypoints; update tests accordingly.

Requirements and context

• No intentional behavior changes unless a bug is discovered.
• Improve supportability.

Suggested execution

• Fork the repo and create a branch: git checkout -b chore/settlement-errors
• 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-settlement (adjust -p if the crate name differs).

Deliverables

• lib.rs + test.rs.

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

chore(settlement): standardize panic strings

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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[KayProject]

@KayProject has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hi maintainer can i work on this

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

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