chore(tests): checks function return type

[JakubOnderka]

Summary

Resolves #1638

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

cargo test

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please
  run dd-rust-license-tool write and commit the changes. More details here.

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[chatgpt-codex-connector]

Match as_kind_id's cfg to the test utilities

This helper is gated only by cfg(test), but it both imports crate::compiler and is now called from compiler::test_util, which is compiled under cfg(any(test, feature = "test")). That breaks feature-specific builds: cargo test --no-default-features --features value compiles this method without the compiler module, while crates enabling features = ["compiler", "test"] can expand test_function! without Value::as_kind_id being present. Gate the helper on the same compiler/test-util configuration or move the mapping into test_util.

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