fix(web-components): recalculate navigation group height on expand

[sylvesterkaczmarek]

Summary of the changes

Fixes grouped navigation item spacing when collapsedIconLabels causes taller items in a collapsed side navigation and the side navigation is then expanded.

IcNavigationGroup cached the sum of child item heights while collapsed. On the first transition to the expanded state, the cache-miss path waited for layout to settle but then reused the old cached total instead of measuring the now-expanded items. This left the grouped-links wrapper at the collapsed height.

This change:

• centralises the current grouped-item height measurement;
• performs the initial measurement after the existing layout debounce instead of before it;
• remeasures after the first side-navigation expansion or collapse for an uncached state;
• preserves the per-state height cache for subsequent transitions.

No public API changes.

Related issue

Closes #3901

Testing

• Adds a unit regression with different collapsed and expanded child heights.
• Verifies the grouped-links height changes from 160px collapsed to 80px expanded.
• Verifies the remeasurement occurs after the existing 100ms layout debounce.
• Final branch is one web-components commit.
• Branch is based directly on develop.

Full upstream CI will run when the PR is opened.

[GCHQ-Developer-530]

Tested using playground, looks good to me!

For other reviewers:
The bug -

Screen.Recording.2026-08-28.at.16.14.52.mov

After this fix -

Screen.Recording.2026-08-28.at.16.14.16.mov