feat(web-components): support flex item dividers

[sylvesterkaczmarek]

Summary of the changes

Add a flexItem prop to IcDivider so vertical dividers can stretch correctly when used as children of flex containers without an explicit height.

When enabled on a vertical divider, the host uses align-self: stretch and the divider line and labelled segments grow to fill the available height. Existing behaviour remains unchanged by default.

A focused unit test verifies the prop applies the flex-item state.

Related issue

Closes #4075

Validation

• flexItem defaults to false
• Vertical flex-item dividers stretch within an intrinsically sized flex container
• Labelled vertical dividers can grow their line segments
• Existing divider behaviour is unchanged when the prop is omitted
• Unit regression coverage added

[GCHQ-Developer-530]

Hi @sylvesterkaczmarek, thanks for making these changes. Just a few comments from me.
Please could you squash all of your web-components commits into one?
Also I think this change would benefit from a Cypress visual regression test so that we can see the prop in action. Would you be able to add one and commit it with the react scope?
Additionally please could you add a story to demonstrate this change both for web-components and react?
Thank you

[sylvesterkaczmarek]

@GCHQ-Developer-530 Thanks. I've squashed the web-components changes into one commit, added a React-scoped Cypress visual regression test with its generated baseline, and added flex item stories for both Web Components and React.

[lz405]

Hi, thanks for the contribution. We generally keep Cypress tests in one file per component.

[sylvesterkaczmarek]

Thanks for the guidance. I've moved the Cypress coverage into the existing IcDivider test file and moved the baseline to match. The web-components and React changes remain separated by scope. The new Actions runs are currently waiting for maintainer approval.

[GCHQ-Developer-530]

Please rebase this branch