Replace the overloaded RequestsMustBePositive error used by set_service_price

[mikewheeleer]

Harden set_service_price to reject negative prices with a precise error

Description

In contracts/escrow/src/lib.rs, set_service_price rejects a negative price_stroops by panicking with EscrowError::RequestsMustBePositive (#2) — an error whose name and doc comment are about record_usage request counts, not prices. This semantic mismatch will confuse client SDKs decoding the error and any operator reading logs. 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::InvalidPrice variant (next free code, append-only) and use it for the negative-price guard in set_service_price.
• Do not renumber or reuse existing codes; #2 keeps its record_usage meaning.
• Update the doc comment on set_service_price and the error enum accordingly.
• Confirm zero price remains allowed (free service) per the existing contract.

Suggested execution

• Fork the repo and create a branch
• git checkout -b security/contracts-25-invalid-price-error
• Implement changes
  • Write code in: contracts/escrow/src/lib.rs — new error variant + guard swap.
  • Write comprehensive tests in: contracts/escrow/src/test.rs — negative price panics with the new code, zero allowed, positive round-trips.
  • 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.
• Test and commit

Test and commit

• Run cargo fmt --all -- --check, cargo build, and cargo test.
• Cover edge cases: -1, i128::MIN, 0, large positive price.
• Include the full cargo test output and a short security notes section in the PR description.

Example commit message

fix: use a dedicated InvalidPrice error for negative set_service_price

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.