feat(go): add DefineSchemasFor and drop core.DefineSchemaFor

[apascal07]

Ports DefineSchemasFor from the Go V2 branch (#5814). Registering the schemas a .prompt-using app needs took one DefineSchemaFor[T] statement per type, since a type parameter cannot be variadic. DefineSchemasFor takes values instead, so one call covers as many types as the app has.

Before: one statement per type.

genkit.DefineSchemaFor[JokeRequest](g)
genkit.DefineSchemaFor[Joke](g)
genkit.DefineSchemaFor[RecipeRequest](g)
genkit.DefineSchemaFor[Recipe](g)
genkit.DefineSchemaFor[AssistantRequest](g)

After: one call. The registry name and the schema are still inferred from each value's Go type, so .prompt frontmatter referencing schema: Joke resolves exactly as before.

genkit.DefineSchemasFor(g, JokeRequest{}, Joke{}, RecipeRequest{}, Recipe{}, AssistantRequest{})

core.DefineSchemaFor is deleted, genkit.DefineSchemaFor stays

core is plugin and internal plumbing, so its copy goes rather than becoming a shim, following ai.DefineFormat in #5861. genkit.DefineSchemaFor is the app-facing touchpoint and is not deprecated: for a single type, naming it still reads better than constructing a value of it. It now delegates to core.DefineSchemasFor, so the two forms cannot drift.

Taking a value rather than a type parameter also lets registration reject input that never produced a usable schema. DefineSchemaFor[map[string]any] and DefineSchemaFor[struct{ ... }] registered under the empty name, and DefineSchemaFor[SomeInterface] dereferenced a nil reflect.Type. All three now panic with a message pointing at DefineSchema(name, schema), which is the call that can name a schema explicitly. That is the only behavior change to the kept function.

Scope

• DefineSchema(g, name, schema) is untouched. A raw JSON schema map has no Go type to take a name from, which is why passing one to DefineSchemasFor panics instead of guessing.
• No registry or .prompt change. Schemas land in the same registry under the same names; only the number of statements it takes to get them there changes.

Testing

Go 1.26.3, full go test ./... in go/: 40 packages pass, no failures. go/samples builds and vets clean.

core_test.go covers multi-value registration, pointer values, and each of the three panic paths. genkit_test.go keeps DefineSchemaFor coverage for both value and pointer type arguments, pinning it to the same schemas DefineSchemasFor produces.

[gemini-code-assist]

Code Review

This pull request introduces DefineSchemasFor, a new function that allows registering multiple JSON schemas derived from Go types in a single call. It refactors existing code, documentation, and samples to use this new variadic function instead of multiple sequential calls to DefineSchemaFor. Additionally, comprehensive unit tests have been added to verify its behavior, including panic conditions for invalid inputs like maps, unnamed types, and nil values. There are no review comments, so I have no additional feedback to provide.