…ompt schemas

Replace the Zod-specific AnySchema type with StandardJSONSchemaV1 from the
Standard Schema spec for user-provided tool and prompt schemas. Enables any
schema library implementing the spec (Zod v4, Valibot, ArkType) to be used.

- New packages/core/src/util/standardSchema.ts with spec interfaces and utils
- RegisteredTool/RegisteredPrompt accept StandardJSONSchemaV1
- Tool callbacks use StandardJSONSchemaV1.InferOutput<T> for inference
- New ArkType and Valibot examples
- New integration tests for mixed schema libraries

completable() remains Zod-specific (relies on .shape introspection).

Co-authored-by: Matt Carey <mcarey@cloudflare.com>

mentioned this pull request

…w fixes

Follow-up to the Standard Schema feature. Completes the schema-agnostic
surface and addresses review findings.

Scope completion:
- Narrow registration types (inputSchema/outputSchema/argsSchema) from
  StandardJSONSchemaV1 to StandardSchemaWithJSON — the SDK needs both
  ~standard.validate and ~standard.jsonSchema, so the type should say so.
  validateStandardSchema drops to 11 lines, no branches.
- Drop resultSchema from experimental.tasks.getTaskResult() — same
  pattern #1606 used for callTool. Returns GetTaskPayloadResult.
- Delete 7 dead exports from schema.ts orphaned by the feature.
- Add fromJsonSchema(schema, validator) adapter for raw JSON Schema
  input (TypeBox, hand-written schemas). Wraps as StandardSchemaWithJSON
  using the SDK's existing AJV/cfWorker validators.

Bug fixes:
- Include issue.path in validation error messages (was dropping the
  field path, e.g. "must be a number" instead of "config.retryCount:
  must be a number").
- Unwrap optional schema in handlePromptCompletion — registration
  checked isCompletable after unwrapping, request handler didn't.
  completable(z.string(), cb).optional() registered but silently
  returned empty. Pre-existing on main.
- Null-safe isOptionalSchema so unwrapOptionalSchema(undefined) doesn't
  crash when the arg name doesn't exist in the schema shape.

Docs and tests:
- Migration guides document the removed exports and use z.object()
  consistently.
- standardSchema.test.ts cleaned to v2 request() API (no schema arg;
  auto-resolved from method name).
- Changeset + tests for fromJsonSchema.

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