fix the diagnosis of typing error of decorator is very hard to read. #1115

[asukaminato0721]

Summary

Fixes #1115

Fixed union alias display so non-generic union type aliases keep their name in nested type positions, which makes decorator error messages readable.

Test Plan

add tests to check this.

[Copilot]

Pull request overview

This PR fixes the readability of decorator error messages by preserving union type alias names in nested type positions. Previously, union type aliases like ResponseReturnValue = int | str | bytes | dict[str, int] | list[int] would be fully expanded in error messages, making them extremely difficult to read. Now, non-generic union type aliases maintain their names, resulting in more concise and readable error messages.

Changes:

• Modified wrap_type_alias in solve.rs to set display names on non-generic union type aliases
• Added test to verify union alias names are preserved in callable return types
• Added test to verify decorator error messages use union alias names instead of fully expanded types

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pyrefly/lib/alt/solve.rs	Added logic to set display_name on non-generic union type aliases and their Type-wrapped variants
pyrefly/lib/test/type_alias.rs	Added test verifying union alias names appear in reveal_type output for callable return types
pyrefly/lib/test/decorators.rs	Added test verifying decorator error messages use union alias names for better readability

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[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅