Correct IsSynchronized values across ListBox collections

[KSaiSivani]

Fixes #3088

Proposed changes

• Override IsSynchronized in ListBox.SelectedIndexCollection and ListBox.IntegerCollection to return false.
• Ensure consistency across all ListBox collections (SelectedObjectCollection, ObjectCollection, SelectedIndexCollection, IntegerCollection).
• Add unit tests verifying that all four collections return IsSynchronized == false.

Customer Impact

• Developers will no longer be misled into thinking SelectedIndexCollection and IntegerCollection are thread‑safe.
• Clearer, consistent API behavior across all ListBox collections.
• Prevents incorrect assumptions about synchronization and reduces risk of threading bugs.

Regression?

• No.

Risk

• Low. The change only affects the IsSynchronized property return value.
• No runtime behavior or collection logic is altered.
• Unit tests ensure correctness and prevent regressions.

Test methodology

• Added unit tests for all four ListBox collections.
• Verified consistent IsSynchronized == false return values.
• Ran full test suite to confirm no regressions.

Test environment(s)

• 11.0.100-preview.3.26170.106

Microsoft Reviewers: Open in CodeFlow

[LeafShi1]

LGTM! @KlausLoeffelmann Please take a look on this

[ricardobossan]

Also LGTM. Issue #3088 discussion already endorses this direction: JeremyKuhne said to make it match the underlying (non-synchronized) behavior; weltkante warned that forwarding IsSynchronized == true is only safe with conditional locking, so false is the safe/correct value here.

[KSaiSivani]

@dotnet-policy-service agree company="Syncfusion, Inc."