Fix integer overflow in string.IndexOf range validation

[prozolic]

Closes #128192
This PR change string.IndexOf range validation from count + startIndex > Length to count > Length - startIndex to avoid the overflow.
This ensures an ArgumentOutOfRangeException is thrown when startIndex + count exceeds int.MaxValue when comparisonType is StringComparison.OrdinalIgnoreCase.

Also applied the same fix to IndexOf(Rune, int, int, StringComparison) and updated it to use ArgumentOutOfRangeException.ThrowIfNegative and ArgumentOutOfRangeException.ThrowIfGreaterThan consistently with the char overload.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates string.IndexOf argument validation to avoid overflow and produce more accurate ArgumentOutOfRangeException parameter names, with added regression tests.

Changes:

• Adjusted startIndex/count range checks to use count > Length - startIndex instead of startIndex + count > Length.
• Added tests asserting correct ArgumentOutOfRangeException parameter names for IndexOf overloads with StringComparison.OrdinalIgnoreCase and Rune.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/StringTests.cs	Adds regression tests for argument validation/paramName behavior in IndexOf overloads.
src/libraries/System.Private.CoreLib/src/System/String.Searching.cs	Fixes validation to be overflow-safe and to surface correct failing parameter names (startIndex vs count).

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[MihaZupan]

Thanks

Looks like tons of these methods also skip all arg validation if the string is empty for some reason.

Btw you can do Closes #123 in the PR description so that the issue gets automatically closed when a PR is merged

[prozolic]

Btw you can do Closes #123 in the PR description so that the issue gets automatically closed when a PR is merged

@MihaZupan Thank you! I didn't realize that's how Closes works.