feat(swift): add final classes option

[coyaSONG]

Description

Add an opt-in --final-classes Swift renderer option. When enabled, it emits every generated Swift reference type as a final class:

• models explicitly emitted as classes via --struct-or-class class
• models promoted to classes to break recursive value-type cycles
• the generated JSONNull, JSONCodingKey, and JSONAny Codable helpers

The option defaults to false, so existing Swift output remains unchanged. Struct output is unchanged in either mode.

Related Issue

Closes #1241

Motivation and Context

Final classes communicate that generated models are not intended as subclassing surfaces and can avoid dynamic subclass dispatch. Making the behavior opt-in provides that benefit without introducing a breaking change for users who currently subclass generated types.

The renderer previously had no option for this behavior: it selected only class or struct for model declarations and hard-coded bare class declarations for helper types.

Previous Behaviour / Output

class Person: Codable {
    // ...
}

class JSONAny: Codable {
    // ...
}

New Behaviour / Output

With --final-classes:

final class Person: Codable {
    // ...
}

final class JSONAny: Codable {
    // ...
}

How Has This Been Tested?

• focused final-class tests: 3/3 passed, including default-off compatibility
• npm run test:unit: 17 files and 130 tests passed with no type errors
• FIXTURE=swift npm run test:fixtures: all 241 Swift fixture runs passed with Swift 6.2.3, including the explicit struct-or-class: class, final-classes: true variant
• npm run build
• Biome on all four changed files
• git diff --check
• manual CLI generation confirmed bare class by default and final class with --final-classes

The repository's requested Node 24.6.0 was not installed in this environment; validation used Node 25.6.1. Implementation and validation were assisted by OpenAI Codex and reviewed against the repository guidance.

Screenshots (if appropriate):

Not applicable; this changes generated source output.

[schani]

Could you put this behind an option that's default off please? It's a breaking change otherwise.