Support exactOptionalPropertyTypes

[haines]

Closes #1358

This PR adds support for the exactOptionalPropertyTypes TypeScript compiler option, which is on by default in tsc --init from Typescript 5.9 onwards. This necessitates adding | undefined to the typing of optional fields so that they may be explicitly set to undefined, not just omitted.

I've enabled the option in the compiler settings for this project to catch any issues, which meant adding a few | undefineds to types used internally as well.

I didn't add it to the JSON types because doing so means either adding | undefined to the fields of JsonObject (which seems wrong when deserializing from JSON can never produce an undefined field) or removing the constraint that MessageJsonType extends JsonValue (which seems wrong because the JSON types are JSON values).

[timostamm]

Thanks for the PR, Andrew! I'm a bit distracted by other things right now, but I'll check it out soon.

[timostamm]

Thanks for the PR, Andrew 🙂

I was afraid this would entail more changes in our runtime packages, but this all looks reasonable to me. I did push up a small change to toJson and fromJson, and updated the code snippets in the manual to match the new generated code.

I didn't add it to the JSON types

Agreed.

[Zaczero]

I don't want to sound salty. But... my original PR #1347 was closed incorrectly, then you made me open an issue to reopen that PR, and instead you just resolved the issue yourself without giving a chance to contribute. And not even co-authored-by.