fix: unions where member lacks discriminator

[bennypowers]

Allows discriminated unions where some members lack the discriminator field entirely, as required by projects like custom-elements-manifest.

Example:

  interface WithDiscriminator { type: "A"; value: string }
  interface WithoutDiscriminator { value: number }
  /** @discriminator type */
  type MyUnion = WithDiscriminator | WithoutDiscriminator;

The generated schema uses if/then conditions:

• If discriminator field is present -> match type WithDiscriminator
• If discriminator field is absent -> match type WithoutDiscriminator

Previously this threw "Cannot find discriminator keyword 'type' in type WithoutDiscriminator"
but this is now valid for projects like custom-elements-manifest where some union members naturally lack the discriminator field.

Changes:

• Separate union members into those with/without discriminator field
• Generate if/then conditions for types with discriminator (unchanged)
• Generate single if/then with anyOf for types without discriminator
• Only require discriminator field if all union members have it
• Handle empty properties case when all types lack discriminator