[iOS][globalization] Investigate matchLength differences between Apple mobile and other ICU platform returned by (Last)IndexOf, IsSuffix, IsPrefix APIs

[matouskozak]

matchLength definition:

When this method returns, contains the number of characters of source that matched the desired value. This may be different than the length of value if a linguistic comparison is performed. Set to 0 if value is not found within source.

Due to the applied normalization

runtime/src/native/libs/System.Globalization.Native/pal_collation.m

Lines 185 to 186 in ec118c7

	NSString *searchStrPrecomposed = searchStrCleaned.precomposedStringWithCanonicalMapping;
	NSString *sourceStrPrecomposed = sourceStrCleaned.precomposedStringWithCanonicalMapping;

the matchLength returned by the (Last)IndexOf, IsSuffix, IsPrefix APIs might not be what users expect.

The precomposedStringWithCanonicalMapping converts the string to its NFC form, potentially causing the returned length to be shorter than expected (if a combination of two code points got normalized into a single code point).

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Possible solutions:

• Revisit the implementation of IndexOf and remove the normalization while ensuring correct behavior.
• Introduce a post-search step, where we would traverse the original source string based on the found index and calculate the matchLength.

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