Tighten BaseModel[M any] to BaseModel[M messageType] where messageType
is a sealed union of *schema.Message | *schema.AgenticMessage. This
prevents nonsensical instantiations like BaseModel[int] at compile time
while remaining fully backward-compatible: external packages use the
type aliases BaseChatModel and AgenticModel, and direct instantiation
with the union members still compiles because Go checks structural
satisfaction regardless of constraint export status.
Change-Id: I836012f7a0b7c3964b7e7b7979d982a7e0f8d540
shentong.martin
a19b7e4190