Annotate Transform::transform with #[must_use] and #[inline]

[coderabbitai]

Summary

Transform::transform currently lacks the #[must_use] and #[inline] annotations that were recently added to Next::new. Aligning the annotations will:

1. Encourage callers to use the returned middleware-wrapped service.
2. Hint the compiler to inline small adaptor implementations for performance.

Context

• Requested by @leynos during review of PR Fix compile errors and clippy #11

Acceptance Criteria

• Transform::transform is annotated with #[must_use] (with an explanatory message) and #[inline].
• Documentation updated to reflect the change.