fix(data-table): avoid spurious expand column on page change

[kelsos]

Summary

• RuiDataTable's onPaginate unconditionally ran set(expanded, []) on every page change. For consumers that hadn't bound v-model:expanded, this wrote into useModel's local cache and flipped expandable (!!get(expanded)) to true, causing useTableColumns to auto-append a non-functional expand column (chevron with no expanded-item slot to render).
• Guarded the reset with if (get(expanded)) so consumers that opt out of expansion are unaffected; consumers that opt in still get pagination resetting their expanded rows.

Test plan

• Unit: new RuiDataTable.spec.ts test "should not emit update:expanded on page change when expanded is not bound" — emits update:modelValue on the inner RuiTablePagination and asserts no update:expanded fired and no [data-id="expand-button"] appears. Verified to fail without the fix.
• E2E: new data-table.spec.ts test "should not introduce an expand-button column on page change when expanded is not bound" — paginates the basic table and asserts zero expand buttons. Verified to fail without the fix.
• Existing 96 RuiDataTable.spec.ts unit tests still pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.80%. Comparing base (39a3e9f) to head (61b6d4c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #525   +/-   ##
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  Coverage   84.80%   84.80%           
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  Files         145      145           
  Lines        5259     5260    +1     
  Branches     1570     1571    +1     
=======================================
+ Hits         4460     4461    +1     
  Misses        799      799           

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