Fix compilation failure from regex source generator with some lazy loops

[stephentoub]

Because of some possible complicated control flow, the compiler thinks these variables are not deterministically initialized.

This repros back to .NET 7 when the source generator was first introduced. It's just a really rare set of constructs that triggers this and so we haven't seen it come up before.

[Copilot]

Pull Request Overview

This PR fixes a compilation failure in the regex source generator that occurs with specific lazy loop constructs. The issue stems from the compiler's inability to determine that certain variables are definitively initialized due to complex control flow patterns in generated code.

• Moves variable declarations for lazy loop position tracking variables to a more appropriate scope
• Adds comprehensive test cases to verify the fix works for problematic regex patterns with lazy loops and alternations

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs	Fixes variable initialization issue by moving declarations to additionalDeclarations and separating assignment
src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Match.Tests.cs	Adds test cases for regex patterns that previously caused compilation failures

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