chore: improve tower maintenance path

[oab24413gmai]

Summary:

• Add unit tests for the Timeout middleware covering the elapsed-error branch: verify that when the inner future exceeds the configured duration, the service returns an Elapsed error (downcastable via the boxed error), and that requests completing before the deadline pass through unchanged. The timeout module files are small and self-contained; tests can be added inline with #[cfg(test)] using tokio::time::pause for deterministic timing.
• Keep the change narrow so it is straightforward to review.

Notes:

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