fix: Auto-scroll crashes when target unmounts mid-scroll — re-check mounted after await

[Fellmonkey]

Description

scrollIntoView() only checked _mounted before
await Scrollable.ensureVisible(_context). After the await,
_waitForScrollToSettle() read _context without re-checking mounted, so a
target that unmounted during the scroll (e.g. the page rebuilt after async
data arrived) crashed with:

This widget has been unmounted, so the State no longer has a context

This PR re-checks _mounted after each await and bails out of the scroll
instead of touching the defunct context.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #662 (also relates to the earlier #313)

Note: test/, docs and examples stays unchecked — this repo has no
test suite yet, and the fix touches private methods only (no doc API), so
no dartdoc/examples change applies.