Fix Ctrl+D in regex find mode

[KryptosAI]

Fixes #107090.

Summary

• preserve isRegex when the multicursor session is created from the active find state
• use that regex flag for next/previous/select-all lookups in MultiCursorSession
• add a regression test covering repeated AddSelectionToNextFindMatchAction calls while the find widget owns a regex search

Validation

• npm run transpile-client
• browser multicursor regression slice in Chromium covering #107090, #5400, #6661, and #20651

[KryptosAI]

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