feat(item): add inner and container parts (#30927)

Issue number: N/A

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## What is the current behavior?
The inner structural elements of item are not exposed as shadow parts, preventing users from being able to customize their styles directly.

## What is the new behavior?
- Exposes `inner` and `container` shadow parts
- Adds e2e test coverage for customizing the shadow parts
- Removes css variables test, combining it with the shadow parts test

## Does this introduce a breaking change?
- [ ] Yes
- [x] No

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Co-authored-by: Brandy Smith <6577830+brandyscarney@users.noreply.github.com>