Splitter - set initial size for collapsed panels

[dmlvr]

No description provided.

[Copilot]

Pull request overview

This PR updates the internal Splitter sizing logic so panes that start in collapsed: true can restore their initially configured size when expanded, and adds/adjusts QUnit coverage in the Splitter widget test suite.

Changes:

• Capture configured size for initially-collapsed panes and use it when calculating the expand delta (instead of always expanding to “half of neighbor”).
• Export Splitter CSS class constants from the internal implementation and reuse them in tests.
• Add new QUnit tests covering size restoration for initially-collapsed panes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
packages/devextreme/js/__internal/ui/splitter/splitter.ts	Captures initial sizes for collapsed panes and uses them on first expand; exports internal CSS class constants.
packages/devextreme/testing/tests/DevExpress.ui.widgets/splitter.tests.js	Imports internal CSS class constants; adds tests for restoring configured size after expanding initially-collapsed panes.

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