Description

In contract_helpers.rs, replaced the ContractIdConstantNotAConstDecl error with an Internal compiler error since this represents an impossible state that should never occur in normal operation.
In error.rs, removed the now unused ContractIdConstantNotAConstDecl error variant and its corresponding span implementation.
The changes make sense because:
The error was only used in one place where it represented an impossible state
If that state is ever reached, it indicates a bug in the compiler itself rather than user error
Using an internal compiler error will make it clearer that this is a compiler bug that needs to be fixed if it ever occurs
fix #6841

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.