test: add regression test for minimum handle length

[shadrach68]

Summary

This PR adds regression testing to cover the minimum boundary of the handle validation during creator registration. While the maximum and empty-string boundaries were previously tested, this ensures that the shortest allowed handle is accepted without error.

Changes

• Added minimum length success test: Registers a creator with a handle of exactly HANDLE_LEN_MIN characters and asserts that the registration succeeds and the state is updated correctly.
• Added below-minimum length failure test: Attempts to register a creator with a handle of HANDLE_LEN_MIN - 1 characters and asserts it is cleanly rejected with ContractError::HandleTooShort.
• Dynamic Constant Reference: Both tests import and reference the HANDLE_LEN_MIN constant directly from the contract to size the byte arrays dynamically, avoiding hardcoded values.

Related Issues

Closes #367

Checklist

• Happy-path test exists for the exact minimum valid length.
• Failure path asserts no unintended state is stored when the handle is too short.
• Tests use contract_test_env helpers where possible.
• Code passes cargo test --workspace locally.

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