Add `enum_field_rename` option in build.yaml for default enum value renaming

### Summary Adds an **`enum_field_rename`** option to the `json_serializable` builder config so packages can set a default field-rename strategy for enum values (e.g. PascalCase from the backend) without annotating every enum with `@JsonEnum(fieldRename: ...)`. Fixes [#1021](https://github.com/google/json_serializable.dart/issues/1021). ### Problem `field_rename` in `build.yaml` only affects **class fields**, not **enum value names**. Enums had to set `@JsonEnum(fieldRename: ...)` on each type. There was no way to configure a package-wide default for enum value renaming. ### Solution Introduce a separate option **`enum_field_rename`** in the build config. When generating enum value maps: - If `@JsonEnum(fieldRename: ...)` is set to something other than `none`, that value is used. - Otherwise, the build config **`enum_field_rename`** is used (default `none`). ### Changes **json_annotation** - Added optional `enumFieldRename` to `JsonSerializable` (used when parsing `build.yaml`). - Updated generated `json_serializable.g.dart` for `fromJson`/`toJson` and allowed keys. **json_serializable** - **ClassConfig**: Added `enumFieldRename` (default `FieldRename.none`) and wired it from `JsonSerializable` and in `mergeConfig`. - **enum_utils**: `enumValueMapFromType` (and helpers) accept optional `defaultEnumFieldRename`; effective rename is annotation value if non-`none`, else config default, else `none`. - **JsonEnumGenerator**: Takes `Settings` and passes `config.enumFieldRename` as the default for enums. - **EnumHelper / encoder_helper**: Pass `context.config.enumFieldRename` (or `config.enumFieldRename`) into enum helpers so the default is applied when encoding/decoding enum fields. ### Examples **1. build.yaml – default PascalCase for all enums** ```yaml targets: $default: builders: json_serializable: options: enum_field_rename: pascal ``` Enums without `@JsonEnum(fieldRename: ...)` (or with `fieldRename: none`) will serialize with PascalCase: ```dart // No per-enum annotation needed enum Status { active, pending, completed } // Serializes as: "Active", "Pending", "Completed" ``` **2. Per-enum override** ```dart @JsonEnum(fieldRename: FieldRename.snake) enum ApiStatus { active, pending } // Uses snake_case: "active", "pending" (annotation overrides build config) ``` **3. Same options as other build config** ```yaml targets: $default: builders: json_serializable: options: field_rename: snake # class fields → snake_case enum_field_rename: pascal # enum values → PascalCase ``` **4. Supported values** Same as `field_rename`: `none`, `kebab`, `snake`, `pascal`, `screamingSnake`. ### Documentation - **README.md** and **tool/readme/readme_template.md**: Document `enum_field_rename: none` in the build configuration options. - In YAML use **snake_case** (`enum_field_rename`); in Dart use **camelCase** (`enumFieldRename`), consistent with `field_rename` / `fieldRename`. ### Testing - **config_test.dart**: `enum_field_rename` in default config, invalid config handling, and key ordering. - **shared_config.dart**: Non-default `enumFieldRename` in `generatorConfigNonDefaultJson`. - **test_sources.dart**: `ConfigurationExplicitDefaults` includes `enumFieldRename: FieldRename.none` so annotation-over-config behavior is tested. - **json_serializable_test.dart**: `JsonEnumGenerator` updated to use `JsonEnumGenerator(Settings())` so enum generation uses the new default. All existing tests pass (624 tests). fixes #1021