Replace the reused NotPendingAdmin error for unauthorized settle and ownership callers

[mikewheeleer]

Add a dedicated Unauthorized error instead of overloading NotPendingAdmin

Description

Both settle and transfer_service_ownership in contracts/escrow/src/lib.rs reject an unauthorized caller by panicking with EscrowError::NotPendingAdmin (#6) — an error whose name and doc comment are specifically about the two-step admin handover in accept_admin_transfer. A code comment even flags the reuse (// reuse: unauthorized caller). Client SDKs decoding #6 cannot tell a failed admin acceptance from a generic unauthorized settle/ownership attempt, and operators reading logs are misled. This issue introduces a precise error while keeping codes append-only.

Requirements and context

• Repository scope: Agentpay-Org/Agentpay-contracts only.
• Add a new EscrowError::Unauthorized variant (next free code, append-only) and use it for the unauthorized-caller branch in both settle and transfer_service_ownership.
• Do not renumber or change the meaning of #6; accept_admin_transfer keeps NotPendingAdmin for its genuine wrong-pending-caller case.
• Update the doc comments on settle and transfer_service_ownership (which currently document the #6 reuse) to reference the new error.
• Keep all valid-path behaviour unchanged.

Suggested execution

• Fork the repo and create a branch
• git checkout -b security/contracts-unauthorized-error
• Implement changes
  • Write code in: contracts/escrow/src/lib.rs — new Unauthorized variant + swap in settle and transfer_service_ownership.
  • Write comprehensive tests in: contracts/escrow/src/test.rs — unauthorized settle and transfer now panic the new code; accept_admin_transfer wrong-caller still panics #6.
  • Add documentation: note the error semantics in README.md.
  • Include NatSpec-style doc comments (///) matching the existing style in lib.rs.
  • Validate security: no code reuse, stable SDK decoding for the admin-handover path.
• Test and commit

Test and commit

• Run cargo fmt --all -- --check, cargo build, and cargo test.
• Cover edge cases: stranger settles (rejected new code), stranger transfers (rejected new code), wrong pending admin accepts (still #6).
• Include the full cargo test output and a short security notes section in the PR description.

Example commit message

security: add dedicated Unauthorized error for settle and ownership callers

Guidelines

• Minimum 95 percent test coverage for impacted modules.
• Clear, reviewer-focused documentation.
• Timeframe: 96 hours.

Community & contribution rewards

• 💬 Join the AgentPay community on Discord for questions, reviews, and faster merges: https://discord.gg/eXvRKkgcv
• ⭐ This is a GrantFox OSS / Official Campaign task and may be rewarded. When your PR is merged you'll be prompted to rate the project — if this issue and the maintainers helped you ship, we'd be grateful for a 5-star rating. Clear questions in Discord and tidy, well-tested PRs are the fastest path to a merge and a reward.

[GBOYEE]

Hello! I can work on this. I'll implement the solution with proper Soroban patterns, auth checks, and comprehensive tests. PR incoming shortly.