[release/9.0-staging][iOS][globalization] Fix IndexOf on empty strings on iOS to return -1

[matouskozak]

Manual backport of #111898.

Fix incorrect return condition for iOS IndexOf implementation and add test case for IndexOf("", <something>) and "".Contains(<something>). Clean-up of some styling issues and adding comments.

Customer Impact

• Customer reported
• Found internally

Customers using CompareInfo.IndexOf or string.Contains APIs on empty source strings involving ICU logic are returning a result 0 (indicates found at index 0) instead of -1 (indicates not found).

Some of the issues reported by customers:

• string.Contains returns wrong result on net-ios with .net 9 #110609
• Incorrect behavior of string.IndexOf in different cultures #111340

Regression

• Yes
• No

The regression was introduced in #86895 and initially limited to when hybrid globalization is enabled. However, in .NET 9, we switched fully to Apple native globalization APIs (initially also referred to as hybrid globalization), thus spreading this issue to all iOS Globalization code using the IndexOf API.

Testing

Previously there were no test cases for this code path. This PR adds test scenarios for both CompareInfo.IndexOf or string.Contains.

Risk

Low

This change is correcting previously incorrect behavior. It is unlikely that .NET 9 iOS customers are depending on this new behavior as it is fundamentally incorrect as per .net doc.

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[matouskozak]

/azp run runtime-extra-platforms

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