Fix Regex source generator multithreading issue

[MihaZupan]

Fixes the issue discussed in #118083 (comment)

Alternatively we could make copies only in IsUnicodeCategoryOfSmallCharCount, but this seemed more likely to prevent us from introducing the same issue from a different caller elsewhere.

[Copilot]

Pull Request Overview

This PR fixes a multithreading issue in the Regex source generator where multiple threads could potentially modify the same character array concurrently. The fix involves making defensive copies of character arrays to prevent race conditions during the sorting operation.

• Changes the EmitSearchValues method signature to accept ReadOnlySpan<char> instead of char[]
• Creates a local copy of the span as an array before sorting to prevent concurrent modifications
• Updates the caller to pass ReadOnlySpan<char> directly instead of pre-converting to array

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[stephentoub]

Thanks.