chore: improve sqlx maintenance path

[oab24413gmai]

Summary:

• Add focused unit tests for the Display/Debug formatting of sqlx_core::error::Error variants (including the configuration, protocol, and io-error wrappers) to lock in the human-readable messages and prevent silent regressions when variants are added or renamed.
• Keep the change narrow so it is straightforward to review.

Notes:

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

[abonander]

The string formatting of errors is not meant to be stable. That's why the enum variants are exposed, so you can match on them.