fix: report unsupported generic ABI declarations

[Soundcreates]

Issue

Closes #7703. Generic parameters in ABI declarations were either accepted without a useful diagnostic or produced confusing downstream errors when the ABI was implemented.

Fix

• Detect generic parameters while converting ABI function declarations.
• Emit a dedicated Generic parameters are not supported in ABI declarations. diagnostic at the generic parameter span.
• Add an end-to-end compile-fail fixture covering the unsupported syntax.

Validation

• cargo test -p sway-core --lib (20 passed).

[fuel-cla-bot]

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[cursor]

PR Summary

Low Risk
Early diagnostic-only change in ABI declaration lowering; no runtime or contract-call behavior changes.

Overview
ABI method signatures with type parameters (e.g. fn fun<T>()) are now rejected during ABI parse-tree conversion instead of slipping through or failing later with unclear errors.

When fn_signature.generics is present on an ABI interface fn, the compiler emits Generic parameters are not supported in ABI declarations. at the generic parameter span via new CompileError::GenericParametersNotSupportedInAbi, wired into the error span mapping like other ABI restrictions.

An e2e should_fail fixture (generic_parameters_in_abi) asserts the diagnostic on fn fun<T>() -> bool;.

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