[Trithem90]

Summary

Fixes ICU-23508.

Deep cloning a UText backed by UnicodeString or UChar* replaced
dest->context with newly owned storage but left dest->chunkContents
pointing to the source storage.

This could cause a deep clone to continue reading from the original buffer
instead of its independent copy.

Fix

Rebind dest->chunkContents to the newly allocated backing storage in:

• unistrTextClone()
• ucstrTextClone()

Tests

Added UTextTest::Ticket23508() covering both affected providers.

The regression test:

• fails for both UChar* and UnicodeString without the fix;
• passes for both with the fix;
• and the full utility/UTextTest suite passes.

Tested with ICU 79.0.1.

[CLAassistant]

All committers have signed the CLA.

[markusicu]

lgtm2 tnx