chore: improve anyhow maintenance path

[vip892766gma]

Summary:

• Add edge-case unit tests for Error::chain() iterator behavior on deeply nested contexts and for context() applied to Result<T, E> where E already implements std::error::Error with a custom Display, verifying the chain order and that downcast_ref still finds the innermost source. Tests-only change, no behavior modification.
• Keep the change narrow so it is straightforward to review.

Notes:

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

[dtolnay]

I think this functionality is covered well enough by the existing tests.