[CLOV-1606][BpkTooltip] Add zIndexValue prop

[Ezreal Yang (Supremeyh)]

Mirror BpkPopover's zIndexValue API on BpkTooltip so consumers can override the floating container's z-index inline.

Default is undefined so the SCSS $bpk-zindex-tooltip token continues to drive the default z-index — keeping the existing behavior, and leaving consumer CSS overrides intact when the prop is not passed.

Remember to include the following changes:

• Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [Clover-123][BpkButton] Updating the colour
• README.md (If you have created a new component)
• Component README.md
• Tests
• Accessibility tests
  • The following checks were performed:
    • Ability to navigate using a keyboard only
    • Zoom functionality (Deque University explanation):
      • The page SHOULD be functional AND readable when only the text is magnified to 200% of its initial size
      • Pages must reflow as zoom increases up to 400% so that content continues to be presented in only one column i.e. Content MUST NOT require scrolling in two directions (both vertically and horizontally)
    • Ability to navigate using a screen reader only
• Storybook examples created/updated
• For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4517 to see this build running in a browser.

[skyscanner-backpack-bot]

	Warnings
⚠️	Package source files (e.g. packages/package-name/src/Component.js) were updated, but snapshots weren't. Have you checked that the tests still pass?

Browser support

If this is a visual change, make sure you've tested it in multiple browsers.

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