[release/9.0-staging] Fix case-insensitive JSON deserialization of enum member names

[github-actions]

Backport of #112028 to release/9.0-staging

/cc @eiriktsarpalis@PranavSenthilnathan

Customer Impact

• Customer reported
• Found internally

When deserializing a string into an enum using a naming policy, if the C# enum member name is the same as its name derived from the naming policy, then the deserialization will be case sensitive. This prevents some values from being deserialized successfully in .NET 9.0 which previously worked fine in .NET 8.0. Reported in #110745.

Regression

• Yes
• No

Introduced in #105032.

Testing

User scenario was validated manually and new unit tests have been added to cover it.

Risk

Low. This is a targeted change that fixes the handling of a corner case that got regressed in .NET 9.

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[jeffhandley]

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[jeffhandley]

@PranavSenthilnathan / @eiriktsarpalis -- I overlooked that this backport had been queued up against release/9.0 instead of release/9.0-staging. I just re-targeted it.