Fix Add Selection to Next Find Match with regex

[Nirajan Banjade (NirajanBanjade)]

Fixes #107090

This fixes "Add Selection to Next Find Match" when regex mode is enabled.

Previously, after selecting a regex match in the editor, the multicursor session could fall back to the literal selected text instead of preserving the regex query.

This change preserves the active regex query when the current editor selection exactly matches that regex in the document.

Testing:

• Added regression tests for regex Find + Add Selection to Next Find Match
• Added coverage for anchored regex behavior
• Verified manually with foo\d matching foo1, foo2, foo3, and foo4
• Verified normal non-regex Cmd+D behavior still works

[Copilot]

Copilot review overview

Review effort: Balanced
Findings: None

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[Nirajan Banjade (NirajanBanjade)]

@microsoft-github-policy-service agree