[pigeon] add error on empty class

[tarrinneal]

just adds an error if a user tries to use an empty class that isn't sealed

List which issues are fixed by this PR. You must list at least one issue.

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[gemini-code-assist]

Code Review

This pull request updates Pigeon to version 27.1.1 and refactors AST validation logic in pigeon_lib_internal.dart to enforce rules on sealed classes, empty classes, and inheritance. It also simplifies field parsing in RootBuilder and adds corresponding tests. The review feedback suggests two improvements to the validation logic: allowing empty subclasses of sealed classes to support idiomatic union types, and checking superClassName to prevent silent validation bypasses when extending undefined classes.

[gemini-code-assist]

In sealed class hierarchies (union types), it is a very common and idiomatic pattern to have empty subclasses to represent states with no associated data (for example, a Loading or Empty state in a Result or Option union).

Currently, the empty class check flags any non-sealed class with no fields as an error. This will incorrectly reject valid empty subclasses of sealed classes.

To support idiomatic sealed class unions, we should allow empty classes if they are subclasses (i.e., superClassName != null).

Suggested change

	if (classDefinition.fields.isEmpty && !classDefinition.isSealed) {
	result.add(
	Error(message: 'Class: "${classDefinition.name}" must contain fields or be sealed.'),
	);
	}
	if (classDefinition.fields.isEmpty && !classDefinition.isSealed && classDefinition.superClassName == null) {
	result.add(
	Error(message: 'Class: "${classDefinition.name}" must contain fields or be sealed.'),
	);
	}

[stuartmorgan-g]

Is Gemini correct here, or is isSealed true for subclasses of a sealed class?

[stuartmorgan-g]

(Either way, we should have a test of this scenario.)

[gemini-code-assist]

If a class extends an unknown or undefined class (e.g., due to a typo or because the parent class is missing from the file), classDefinition.superClass will be null while classDefinition.superClassName is non-null. In this case, the current check classDefinition.superClass != null is skipped entirely, allowing the undefined inheritance to pass validation silently.

To prevent this silent bypass, we should check if superClassName is non-null, and flag an error if superClass is either null (meaning the parent class was not found/defined) or not sealed.

Suggested change

	if (classDefinition.superClass != null) {
	if (!classDefinition.superClass!.isSealed) {
	result.add(
	Error(message: 'Child class: "${classDefinition.name}" must extend a sealed class.'),
	);
	}
	}
	if (classDefinition.superClassName != null) {
	if (classDefinition.superClass == null || !classDefinition.superClass!.isSealed) {
	result.add(
	Error(message: 'Child class: "${classDefinition.name}" must extend a sealed class.'),
	);
	}
	}