fix(splitter): preserve pane sizes after collapse - 21.2.x

[viktorkombov]

Closes #17545

Description

This PR prevents IgxSplitterPane from losing its configured size after being collapsed and expanded.

Previously, collapsing a pane reset its siblings to size="auto", which caused horizontal panes to lose their previous width. The updated behavior:

• Preserves pane sizes and min/max constraints.
• Clears only temporary drag sizes.
• Allows fixed-size siblings to fill the released space while a pane is collapsed.
• Handles both runtime and initially collapsed panes.

Testing

Added coverage for horizontal and vertical splitters, fixed-size panes, initial collapse, and temporary drag-size cleanup.

Motivation / Context

Type of Change (check all that apply):

• Bug fix
• New functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes)
• Documentation
• Demos
• CI/CD
• Tests
• Changelog
• Skills/Agents

Component(s) / Area(s) Affected:

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Test Configuration:

• Angular version:
• Browser(s):
• OS:

Screenshots / Recordings

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified