Port NIM regex tests to improve test coverage

[Copilot]

Fixes #61895

This PR ports regex functional tests from the nim-regex project to increase .NET's regex test coverage, following the same pattern used for PCRE and Rust test suites.

Changes

• Added RegexNimTests.cs with 156 unique test cases focusing on patterns not already covered by existing tests
• Updated System.Text.RegularExpressions.Tests.csproj to include the new test file
• All tests pass across all regex engines (Interpreter, Compiled, NonBacktracking, SourceGenerated)

Test Coverage

The ported tests provide additional coverage for unique patterns including:

• Escaped quantifiers: \*+, \??, \++? - testing literal *, +, ? characters with quantifiers
• Nested repetitions: (a*)*, (a*)*b, ((a)*(a)*)* - edge case patterns
• Complex patterns: a(b|c)*d, ((a(b)*)*(b)*), alternations with grouping
• Unicode character classes: \d matching ۲, \D matching ⅕
• Lookahead/lookbehind: Positive/negative assertions (filtered for NonBacktracking engine)
• Named groups: (?<name>...) syntax
• Word boundary edge cases: \b\b\b patterns
• Empty patterns and special cases

Deduplication

After analysis, 25 duplicate and trivial test cases were removed that were already covered by existing tests in Regex.Match.Tests.cs, Regex.Groups.Tests.cs, and other test files. The remaining tests focus on unique patterns and edge cases.

Compatibility Notes

Some NIM-specific patterns were excluded or adapted for .NET compatibility:

• Removed \y escape sequence (not recognized in .NET)
• Removed (?U) ungreedy flag (not supported in .NET)
• Removed Unicode case folding tests (ß ↔ ss) due to different behavior
• Converted named group syntax from (?P<name>) to .NET's (?<name>) format
• Filtered lookahead/lookbehind tests for NonBacktracking engine (throws NotSupportedException)

Test Results

• Total tests: 29,899 (added ~600+ test executions across all engines)
• Failures: 0
• All tests pass successfully

The MIT license for nim-regex was already present in THIRD-PARTY-NOTICES.TXT.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Port nim regex tests</issue_title>
<issue_description>Nim has lots of regex functional tests we should port over to improve our coverage:
https://github.com/nitely/nim-regex/blob/eeefb4f51264ff3bc3b36caf55672a74f52f5ef5/tests/tests.nim</issue_description>

<agent_instructions>You can see in https://github.com/dotnet/runtime/tree/main/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests we have a file of tests ported from PCRE and a file of tests ported from Rust. Please similarly port the cited NIM tests (or a newer version if there is one); any that don't pass but should need to be marked as [ActiveIssue(...)]... any that don't pass but that's expected should be removed.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@ Tagging subscribers to this area: @eerhardt, @dotnet/area-system-text-regularexpressions See info in [area-owners.md](https://github.com/dotnet/runtime/blob/main/docs/area-owners.md) if you want to be subscribed.

Issue Details

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Nim has lots of regex functional tests we should port over to improve our coverage:
https://github.com/nitely/nim-regex/blob/eeefb4f51264ff3bc3b36caf55672a74f52f5ef5/tests/tests.nim

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	.NET 7.0

@joperezr This will likely miss ZBB but will still make it before 7.0. Adjusting the milestone just because this is not technically a blocker for 7.0.

Fixes #61895

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Pull Request Overview

Ports Nim regex functional tests to expand .NET regex engine coverage and align with existing PCRE and Rust imported suites.

• Adds 181 Nim-derived test cases in a new RegexNimTests.cs file.
• Updates the test project file to include the new test source.
• Tests exercise multiple engines; lookaround tests are conditionally included.

Reviewed Changes

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

File	Description
System.Text.RegularExpressions.Tests.csproj	Adds RegexNimTests.cs to build for inclusion of new test cases.
RegexNimTests.cs	Introduces Nim-sourced regex functional tests across patterns, quantifiers, character classes, anchors, options, and lookarounds.