feat(course-outline): add unit title link in expanded outline view

[abhalsod-sonata]

• Updated TitleButton to support external and internal links, allowing the title to be clickable.
• Added hover and focus styles for linked titles in CardHeader.scss.
• Implemented tests to verify link rendering and interaction behavior in UnitCard.
• Ensured that clicking the title link does not expand the unit card, while the expand button still functions as intended.

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Description

This PR makes the unit title clickable in the expanded Course Outline view in Studio, allowing Course Authors to navigate directly to a unit without first expanding it.

TitleButton now accepts an optional titleLink prop:

• When titleLink is not provided, the button renders exactly as before — clicking anywhere on it toggles expand/collapse.
• When titleLink is provided:
  • The title text renders as a link — Hyperlink for absolute/external URLs, or a react-router-dom-backed Button (as={Link}) for internal routes — determined by a new isExternalTitleLink check (/^[a-z][a-z\d+\-.]*:/i).
  • A separate expand/collapse Button is overlaid across the full row (position-absolute, w-100 h-100, transparent background), so clicking anywhere except the title link toggles the unit's expanded state, while clicking the title navigates to the unit.
  • Both render paths now share a single ExpandIcon variable (previously an inline isExpanded ? ArrowDownIcon : ArrowRightIcon ternary) and a shared titleText element.

New --with-link modifier styles in CardHeader.scss:

• Highlights the row background on hover/focus-within.
• Underlines the title text on hover.
• Adds a :focus-visible outline on the expand control for keyboard users.
• Ensures the pgn__icon doesn't intercept pointer events, and fixes truncation (truncate-1-line) for the linked title content.

UnitCard.tsx passes titleLink={getTitleLink(id)} to TitleButton, wiring this new behavior into unit cards in the expanded outline view.

User roles impacted: Course Author (Studio – Course Outline page).

Supporting information

• Ticket: LP-249

Testing instructions

1. Pull this branch and run Studio.
2. Open a course's Course Outline page and switch to the expanded outline view.
3. Expand a section/subsection so unit cards are visible.
4. Hover over a unit's title — the row background should highlight and the title text should underline.
5. Click the unit's title text:
   • You should navigate to that unit.
   • The unit card should not expand to show its components.
6. Click elsewhere on the unit row (the expand/chevron area):
   • The unit card should expand/collapse as before.
7. Tab to the row with the keyboard and confirm a visible focus outline appears on the expand control.
8. Run:

   jest src/course-outline/unit-card/UnitCard.test.tsx
   

   and confirm the three new tests pass:
   • renders the unit title as a link when expanded outline view is enabled
   • does not expand the unit card when the title link is clicked
   • expands the unit card when the non-title expand area is clicked

Other information

• No backend/API changes — getTitleLink(id) already existed; this PR wires it into TitleButton.
• No new dependencies — Hyperlink, OverlayTrigger, Stack, and Tooltip are existing @openedx/paragon components already used elsewhere in this codebase.
• Accessibility: avoids nesting an interactive link inside an interactive button by overlaying the expand button behind the title link and stopping click/mousedown propagation on the link, so the two controls don't conflict. Added :focus-visible styling on the expand control.
• This PR targets release-ulmo rather than the default branch — double-check whether it also needs to land on the main development branch.

Best Practices Checklist

• Any new files are using TypeScript (.ts, .tsx) — N/A, no new files added.
• Deprecated propTypes, defaultProps, and injectIntl patterns are not used in any new or modified code.
• Tests use the helpers in src/testUtils.tsx (initializeMocks).
• No new fields added to the Redux state/store.
• Use React Query to load data from REST APIs — N/A, no new API calls in this PR.
• All new i18n messages have a description for translators — N/A, no new i18n messages added.
• Imports avoid using ../.