feat(go/core): add options-struct New*ActionOf constructors and delete registry-taking Define* helpers

[apascal07]

Reworks the core action constructor surface as a non-breaking addition for the next minor release: options-struct constructors under new names (New*ActionOf), deprecated old constructors delegating to them, and outright deletion of the registry-taking Define* helpers, which have no reachable external callers. This is the base PR of a train: the typed-config primitive constructors (ai.NewTyped*) build on it next, followed by plugin migrations.

Options-struct action constructors

The core constructors gain variants that lead with the action type (matching ResolveActionFor, which already leads with it) and take one options struct covering all schema slots. The names read as "new action of <type>".

Before: metadata and the input schema are positional, and output and stream schemas have no override at all.

core.NewAction[In, Out](name, atype, metadata map[string]any, inputSchema map[string]any, fn)
core.NewStreamingAction[In, Out, Stream](name, atype, metadata, inputSchema, fn)
core.NewBackgroundAction[In, Out](name, atype, metadata, startFn, checkFn, cancelFn)
core.NewBidiAction[In, Out, Stream, Init](name, atype, opts, fn)

After: all schema slots share one inference and override path.

type core.ActionOptions struct {
    Description  string
    Metadata     map[string]any
    InputSchema  map[string]any // inferred from In if nil
    OutputSchema map[string]any // inferred from Out if nil
    StreamSchema map[string]any // inferred from Stream if nil
}

core.NewActionOf[In, Out](atype, name, opts, fn)
core.NewStreamingActionOf[In, Out, Stream](atype, name, opts, fn)
core.NewBackgroundActionOf[In, Out](atype, name, opts, startFn, checkFn, cancelFn)
core.NewBidiActionOf[In, Out, Stream, Init](atype, name, opts, fn)

core.BackgroundActionDef is renamed to BackgroundAction so the concrete action type names are consistent (Action, BackgroundAction, BidiAction, Flow); the Def suffix carried no distinction since the action interface already lives at api.Action. The old name remains as a deprecated alias, mirroring the existing ActionDef alias.

Options structs hold descriptor data only (documented on ActionOptions); implementation functions are positional, with optional ones nil-able, so the background cancel function stays a trailing argument. NewBackgroundActionOf takes its own data-only BackgroundActionOptions (ActionOptions minus the stream slot, since the component actions are non-streaming) and no longer leaks the start action's schema slots into the check/cancel sub-actions; they keep the shared description and metadata but infer their own Operation schemas. The bidi constructors already took an options struct, so their Of variant only changes the argument order; BidiActionOptions gains Description to match ActionOptions.

All framework-internal call sites (flows, tools, prompts, resources, generate, agent snapshot/abort actions) now use the new constructors.

New shared helpers back this and the follow-up typed-config work, reusable by in-repo plugins: base.ConvertToExact (exact-type conversion with base.ErrTypeMismatch) and base.SchemaMapFor[T] (schema inference that returns nil for interface types). core.InferSchemaMap remains the exported value-based helper.

Registry-taking Define helpers deleted

A registry is unobtainable outside the framework: plugins return actions from Init for the framework to register, and applications go through the genkit package. So the registry-taking Define* helpers in core had no reachable external callers and are deleted outright rather than deprecated; the genkit wrappers remain the user touchpoints and now compose New* + Register directly.

Deleted: DefineAction, DefineStreamingAction, DefineBackgroundAction, DefineBidiAction, DefineFlow, DefineStreamingFlow, DefineSchema, DefineSchemaFor.

To make New* + Register fully equivalent to the old Define*, the "dynamic" metadata marker ("created at runtime, outside any registry") is now registration-derived: Register removes it instead of leaving stale construction-time state (pinned by test; nothing in the Go codebase reads the flag).

Deprecations

Deprecated, delegating to the new functions with identical behavior: core.NewAction, core.NewStreamingAction, core.NewBackgroundAction, core.NewBidiAction (each pointing at its New*ActionOf counterpart).

When the deprecated functions are removed in the next major, the new ones can collapse back to the plain names with the same signatures, so migrating now is the last rename.

Scope

• No Define*Of surface. Registration composes through the existing Register methods, so a parallel Define* family would only be New* + Register under a second name. The genkit wrappers stay the registering touchpoint.
• The cancel function stays out of BackgroundActionOptions. Holding a CancelOpFunc[Out] would force the options struct to become generic over Out for one optional field, so it stays a nil-able trailing argument and the struct stays data-only.
• The ai and genkit layers are untouched here. Their constructors move in feat(go/ai): add typed-config NewTyped* constructors #5849, which builds on this; plugin migrations follow that.

Verification

Go 1.26.3, full go build ./... and go test ./... in go/: 40 packages with tests pass, no failures.

New coverage in core/action_options_test.go: each schema slot is inferred when the option is nil and honored when set, Description wins over Metadata["description"] and falls back to it, a streaming action infers its stream schema, the deprecated constructors delegate to their Of counterparts, and Register clears the "dynamic" marker. core/define_test.go holds test-local New* + Register helpers standing in for the deleted registry-taking Define*, so the existing suites keep exercising the same construction paths under the new constructors.

[gemini-code-assist]

Code Review

This pull request refactors the action definition and registration API in Genkit by deprecating flat-argument constructors (e.g., NewAction, NewBackgroundAction) in favor of options-based constructors (e.g., NewActionOf, NewBackgroundActionOf) that accept a new ActionOptions struct. It also removes global Define* helpers in favor of explicit creation and registration (e.g., calling .Register(r) or r.RegisterSchema). Feedback on the changes suggests simplifying the description assignment in go/ai/exp/agent.go by setting it directly on the BidiActionOptions struct instead of using a conditional block.