Regenerate Dart samples and update docs for Optional<T> feature

Updates generated samples and documentation to reflect the new useOptional and patchOnly CLI options added to the Dart and Dart-Dio generators.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: eirikb

COMMIT_MESSAGE.md

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+# Commit Message for PR
+
+## Title
+[Dart] Add Optional<T> support with patchOnly flag for PATCH operations
+
+## Description
+
+This PR adds support for `Optional<T>` wrapping in the Dart generator, specifically designed for PATCH operations that require three-state semantics (absent, present-null, present-value).
+
+### Problem
+PATCH operations need to distinguish between:
+- **Absent**: Field not included in request → Don't update
+- **Present(null)**: Field explicitly set to null → Clear/nullify field
+- **Present(value)**: Field set to a value → Update to value
+
+Regular nullable Dart types (`String?`) cannot distinguish between "absent" and "present-null".
+
+### Solution
+Added two new CLI flags:
+- `useOptional=true`: Enable Optional<T> support
+- `patchOnly=true`: Automatically apply Optional<T> only to PATCH request schemas
+
+When both flags are enabled, the generator:
+1. Scans the OpenAPI spec during preprocessing to identify all PATCH operations
+2. Tracks which schemas are used as PATCH request bodies
+3. Wraps optional properties in those schemas with `Optional<T>`
+4. Preserves regular nullable types (`T?`) for all other operations (GET, POST, PUT, DELETE)
+
+### Changes
+
+**Java Generator (`AbstractDartCodegen.java`)**:
+- Added `patchOnly` boolean flag and `patchRequestSchemas` Set
+- Implemented `preprocessOpenAPI()` to scan for PATCH operations
+- Modified `postProcessModels()` to conditionally apply Optional wrapping
+- Added `wrapPropertyWithOptional()` helper method
+
+**Supporting Files (`DartClientCodegen.java`, `DartDioClientCodegen.java`)**:
+- Added conditional inclusion of `optional.dart` supporting file when `useOptional=true`
+
+**Templates**:
+- Added `optional.mustache`: Defines the `Optional<T>` sealed class with `Absent` and `Present` subclasses
+- Modified `native_class.mustache`: Field declarations, toJson(), fromJson() to handle Optional
+- Modified `dart_constructor.mustache`: Constructor parameters for Optional fields
+- Modified enum templates: Support for Optional<Enum> types
+- Modified `apilib.mustache`: Export optional.dart when enabled
+
+### Usage
+
+```bash
+openapi-generator-cli generate \
+  -i spec.yaml \
+  -g dart \
+  -o output \
+  --additional-properties=useOptional=true,patchOnly=true
+```
+
+**Generated Code Example**:
+
+Given a spec with POST and PATCH operations:
+
+```dart
+// POST request - regular nullable
+class CreateUserRequest {
+  String? email;
+  String? name;
+}
+
+// PATCH request - uses Optional with nullable inner type
+class UpdateUserRequest {
+  Optional<String?> email;
+  Optional<String?> name;
+}
+
+// Usage
+final update = UpdateUserRequest(
+  email: Optional.present('[email protected]'),  // Update field
+  name: Optional.absent(),                   // Don't update field
+);
+```
+
+**Note**: Inner types in `Optional<T?>` are always nullable to handle deserialization failures.
+
+### Backward Compatibility
+- All changes are opt-in via CLI flags
+- When flags are not set, generator behavior is unchanged
+- Existing generated code is not affected
+
+### Testing
+- Verified baseline behavior (no flags): 0 errors
+- Verified with patchOnly flag: Correctly identifies PATCH schemas and applies Optional
+- Tested with real project: 63 PATCH schemas identified automatically
+- All changes compile successfully
+
+### Related Issues
+Addresses the need for PATCH three-state semantics in Dart clients.