[contextgen] Honor parameterization in generator instance, expose parameters to serialize

[sneakers-the-rat]

Builds on #1982

Noticed that the contextgen advertises parameters in its dataclass fields that it then ignores during serialization.

If those fields exist, they should do something!

set all args to default None and use instance attributes as defaults instead of method args as defaults - parameters can be set in the instance body, but overridden when calling serialize

[sneakers-the-rat]

looks like the failing tests are choices to make - the old behavior didn't honor the defaults set in the instance, so the tests confirm that. if we want that to be the case (ie. when you set a value in the generator instance it is actually used) then we need to change those tests probably. since this PR is unsolicited i'll wait until someone confirms this is the desired behavior before fixing