fix: required TypeEnum deserializer should reject null values

[TabishRiazBajwa]

Fixes #1678

Read() in the generated TypeEnum JsonConverter silently accepts null during deserialization even when the field is marked as required/non-nullable. This violates the non-nullable contract — a required enum field should never deserialize to null without throwing.

Description
The generated TypeEnumJsonConverter.Read() returns null when the JSON value is null, regardless of whether the property is required. This means a required TypeEnum field can silently become null at runtime, bypassing the non-nullable contract and causing unexpected null reference exceptions downstream rather than at the deserialization boundary where the error actually is.

Fixed by checking for null in the Read() method of the generated converter and throwing a JsonException when a null value is encountered for a required field.

Tested scenarios

• Required TypeEnum field throws JsonException when JSON contains null
• Optional TypeEnum field continues to deserialize null correctly
• All existing IEnum and TypeEnum deserialization tests continue to pass

Fixed issue: #1678

[gemini-code-assist]

Code Review

This pull request updates the C# templates to prevent null values for required, non-nullable string-based enums during JSON deserialization, throwing a JsonException if encountered, and adds corresponding unit tests. The feedback recommends updating the signature of FromStringOrDefault to accept a nullable string to prevent compiler warnings when Nullable Reference Types are enabled. Additionally, it is suggested to extend this null-checking logic to required, non-nullable numeric enums for completeness.

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