Tearsheet layout updates: Masthead coverage and inset token alignment

[lboehling]

Following design demos and PR feedback, two layout refinements were identified for the Tearsheet component group to maximize vertical screen space and fix horizontal edge alignment with page chrome.

Proposed Updates:

1. Masthead Coverage Support

• Problem: Tearsheets currently display below the masthead, limiting available vertical space compared to large modals.

• Solution: Allow tearsheets to overlap/cover the masthead area by default or via configuration.

• Prop Idea: Introduce a prop like coverMasthead (boolean) or showMasthead (boolean) so consumers can toggle masthead visibility based on workflow needs.

2. Page Chrome Inset Alignment

• Problem: The current 4rem (32px) inset creates an ~8px mismatch against standard page chrome (24px inset via --pf-t--global--spacer--inset--page-chrome), causing a sliver of the underlying page container to show along the right edge.

• Solution: Update width and max-width CSS logic to leverage the design token rather than hardcoded rem values.

• context from @mcoker's comment from the original PR:

  Just an observation (non-blocking IMO) is that the tearsheet inset is about 8px more than our default page chrome/layout inset, so you can see a small bit of the page container behind/along the right edge of the tearsheet.
  This is due to the 4rem inset of the tearsheet creating a 32px (2rem) gap on either side, where the page container behind it uses a 24px inset (comes from --pf-t--global--spacer--inset--page-chrome). I imagine the fix would be something like changing the width/max-width to use calc(100% - var(--pf-t--global--spacer--inset--page-chrome * 4) for stack 0, and calc(100% - var(--pf-t--global--spacer--inset--page-chrome * 2) for stack 1. Though if you do that, it's worth noting you should import the tglobalspacerinsetpage*chrome @react-tokens token for the token var instead of hardcoding it.

@GAUNSD @thatblindgeye

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Jira Issue: PF-4685