chore: improve thiserror maintenance path

[vip892766gma]

Summary:

• Add a small edge-case unit test covering #[error(transparent)] combined with a generic type parameter on the error enum variant, verifying both Display forwarding and source() chain behavior — extends existing test patterns without touching library code.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.