fix(tabs): remove the tab from its parent collection on dispose

[damyanpetev]

IgbTab.Dispose() called ContentTabsCollection.Add(this) — the same call OnInitializedAsync makes directly above it:

public void Dispose()
{
    if (TabsParent != null)
    {
        var sv = (IgbTabs)TabsParent;
        sv.ContentTabsCollection.Add(this);   // Add, on teardown
    }
}

The fix: Add → Remove. This is what the other five parent/child handling have always done — IgbExpansionPanel, IgbDropdownItem, IgbSelectItem, IgbTile and IgbTreeItem all call ContentItems.Remove(this) in Dispose.

Adds three tests: registration on initialize, one child disposed, and all children disposed. The two regression tests were confirmed failing against the bug before the fix.

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

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Checklist:

• [ x] All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code
• This PR includes API docs for newly added methods/properties
• 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 behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified

[Copilot]

Pull request overview

Fixes a lifecycle bug in the Ignite UI for Blazor IgbTab component where disposing a tab incorrectly added it back into its parent IgbTabs collection, causing stale/duplicated tab references to persist after re-renders.

Changes:

• Corrected IgbTab.Dispose() to remove the tab from IgbTabs.ContentTabsCollection instead of adding it.
• Added unit tests validating tab registration on initialization and proper collection updates when one/all child tabs are disposed via re-render.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/components/Blazor/Tab.cs	Fixes teardown logic to remove disposed tabs from the parent tabs collection.
tests/IgniteUI.Blazor.Tests/TabsTests.cs	Adds regression tests covering child tab registration and removal from ActualTabsCollection on disposal.