Error handling migration: Result-based API instead of panic

[greatest0fallt1me]

Description

Evaluate migrating public functions from panic! to Result with contracterror! enum for clearer client errors.

Requirements and context

• Breaking ABI change — version bump and migration guide.
• Prefer phased approach if needed.

Suggested execution

• Fork the repository and create a branch (example below).
• Implement the changes in the StreamPay-Contracts Soroban crate (streampay-contracts).
• Keep the contract secure, tested, and documented; prefer small, reviewable diffs.

git checkout -b feature/contract-error-enum

Implementation targets

• lib.rs — define errors; update tests.
• docs/errors.md.

Tests and validation

• All former panic paths return distinct errors.

• Run cargo test in StreamPay-Contracts.

• Cover edge cases; capture test output and short security notes in the PR description.

Guidelines

• Target minimum 95% test coverage for touched contract code (or justify gaps explicitly).
• Clear documentation (rustdoc / project docs as specified).
• Timeframe: 96 hours from assignment.

Example commit message

feat(contracts): structured errors for stream operations