refactor(core): migrate MessageV2 part leaves + ToolPart to Effect Schema

Migrates all 13 part leaf schemas and the FilePartSource union to Effect Schema with derived .zod compatibility. ToolPart also moves, now referencing ToolState directly via Schema.Struct. The Part discriminated union stays Zod in this slice (migrates next). Inlines the four *PartInput schemas in prompt.ts so the z.discriminatedUnion continues to infer correctly without the .omit/.partial helpers that only existed on the original Zod definitions.

Author: kitlangton