fix: annotate CommonError with #[contracterror] and stable u32 codes

[dzekojohn4]

Summary

CommonError was declared as a plain Rust enum without the #[contracterror] macro, so downstream contracts could not use its variants in Result<T, CommonError> return types. This change wires the macro in and assigns stable discriminant codes.

Changes

• contract/common/src/lib.rs:
  • Add contracterror to the soroban_sdk import.
  • Annotate CommonError with #[contracterror] and #[derive(Copy, Clone, Debug, Eq, PartialEq)].
  • Assign stable u32 discriminants: InvalidInput=1, Unauthorized=2, NotFound=3, AlreadyExists=4, InternalError=5, RateLimited=6, Maintenance=7.
  • Update errors::error_codes constants to match the new discriminants (1, 2, 3).

Testing

• cargo check --workspace passes with the annotated enum.
• Downstream contracts returning Result<T, CommonError> compile without TryFromVal errors.

Closes #515