Notify Flutter inspector when navigating widget tree with keyboard (#9810)

* Notify Flutter inspector when navigating widget tree with keyboard

* Add inspector tree left/right keyboard navigation coverage

Updates inspector tree tests to cover left and right arrow navigation, including expand, next-row selection, and parent selection behavior. Also ensures left navigation to a parent notifies Flutter Inspector consistently with other keyboard selection changes.

* Fix inspector widget tree visibility issues during keyboard navigation

Resolves two regressions surfaced when navigating the inspector widget
tree with the arrow-left key.

1. Clicking a still-visible row used to call `expandPath` on the clicked
   node, which re-expanded the clicked node itself and undid any subtree
   collapses the user had just performed via the arrow-left key. Removed
   the call from `onSelectNode`; programmatic selection flows (search,
   on-device pick) continue to call `expandPath` via `syncTreeSelection`,
   so external selection still works correctly.

2. Collapsing all the way to the root via the arrow-left key shrank the
   visible rows down to a single row, which the inspector treated as a
   "still loading" state and replaced with a spinner — hiding the user's
   `[root]` row. Gated that branch on `!firstInspectorTreeLoadCompleted`
   so the spinner only shows during the initial load; afterwards a
   one-row tree renders as the legitimate single-row state.

Adds regression tests covering both behaviors.

* docs: add release notes for inspector keyboard navigation fixes (#9810)

* test: update inspector tree selection test for collapsed nodes