feat: stabilize code generator

[kansson]

Type System:

• Refactor ConvexFunction from data class to sealed interface
  hierarchy
• Add HttpAction type (filtered from code generation)
• Add Nullable value class for nullable union serialization

Union Support:

• Literal unions (v.literal("a") | v.literal("b")) → Kotlin enums
  with @SerialName
• Nullable literal unions → Nullable
• Primitive unions (string | number) → sealed class with typed
  variants (StringValue, DoubleValue, etc.)
• Object/discriminated unions → sealed class with auto-detected
  @JsonClassDiscriminator
• T | null → Nullable wrapper

Object Handling:

• Empty objects → data object (instead of data class)
• Functions without args → data object
• Field name sanitization with @SerialName for special characters
• Class name sanitization (CamelCase conversion)

DSL Builders:

• Proper handling of nested object builders
• Optional field support with null defaults
• Sealed class fields nullable in builder with !! assertion on
  build()
• Nullable union fields unwrap to inner type in builder