Use $enumEncode for non-nullable enum toJson lookups

[AzazelSensei]

Summary

Generated toJson for non-nullable enums currently emits _$EnumMap[instance.field]!. When that expression is stored in a Map<String, dynamic>, analyzers that enable unnecessary_null_checks flag the bang as unnecessary.

This adds a small $enumEncode helper (alongside $enumDecode) and uses it for non-nullable enum encoding so generated call sites stay lint-clean while still producing a non-nullable value for Map keys and other typed contexts (#1145).

Fixes #1515

Test plan

• dart test --exclude-tags=presubmit-only in json_serializable (660 passed)
• Integration regression for Unnecessary use of a null check ('!') on enum values. #1515 + existing Enum serialization creates nullable type for non-nullable enums #1145 map-key typing test
• Regenerated committed .g.dart outputs

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