Add unit tests for contractErrors.ts parsing

[Ejirowebfi]

Description

contractErrors.ts parses Soroban error codes into human-readable messages, but has no unit tests. A regression here would show raw error codes to users.

Tasks

• Create frontend/src/test/contractErrors.test.ts
• Test each known error code (1–7) maps to the correct message
• Test unknown error codes return a generic fallback message
• Test malformed error strings do not throw

Acceptance Criteria

• All known error codes produce the expected string
• Unknown codes return a non-empty fallback
• No test throws an unhandled exception