fix: Expand a collapsed work item thread when its comment is linked

What does this MR do and why?

Following a same-page link to a comment in a resolved (collapsed) thread left the thread collapsed, so the comment stayed hidden. The target-note hash was read once from a non-reactive computed, and the discussion component only used the expand-on-load prop for its initial state.

This tracks the URL hash in reactive data updated on hashchange, and watches the expand-on-load prop so a thread expands when it becomes the hash target.

Changelog: fixed Closes #632443 (closed)

Before / After

Before: Clicking a link to a comment inside a resolved, collapsed thread scrolled to the thread but left it collapsed, so the linked comment stayed hidden — the hash was read once and the thread only honored the expand prop on its initial render.

After: The URL hash is tracked reactively (updated on hashchange), and the discussion expands when it becomes the hash target, so following a link to a comment in a collapsed thread reveals that comment.

How to set up and validate locally

1. Run the updated specs:

   yarn jest spec/frontend/work_items/components/work_item_notes_spec.js spec/frontend/work_items/components/notes/work_item_discussion_spec.js

2. Or in the UI: on a work item with a resolved (collapsed) thread, open a link to a comment inside it (copy the comment's link and visit it) — the thread expands and the comment is shown.

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Related to #632443 (closed)