Generate typed Go union interfaces

[qmuntal]

Summary

This PR updates the Go code generator and generated Go SDK types to model JSON-schema unions as typed interfaces instead of flattened structs where the schema has a discriminator.

Changes included:

• Adds generator support for interface-based discriminated unions, primitive unions, raw fallback variants, and custom JSON encoding/decoding for interface-valued fields.
• Regenerates Go RPC and session-event types, moving generated JSON helpers into go/rpc/zrpc_encoding.go and go/zsession_encoding.go.
• Makes SessionEvent.Type() derive from SessionEvent.Data and preserves unknown session event types through RawSessionEventData.
• Updates Go session logic, tests, E2E helpers, examples, and image-input docs to use the new concrete union variants and Type() method.

Why this is beneficial

• Prevents invalid union states that were possible with flattened structs containing many optional fields.
• Makes Go APIs more schema-like and type-safe: callers can switch on concrete variants instead of inspecting optional fields that may not apply to a given union member.
• Keeps discriminator values read-only and derived from the concrete type, avoiding mismatches between type/kind fields and payload contents.
• Improves forward compatibility by preserving unknown union and session event payloads instead of dropping or failing on newer schema variants.
• Keeps generated type declarations easier to scan by moving bulky JSON encoding logic into generated z*encoding.go files.
• Supports interface-valued fields in generated types without making SDK callers manually decode raw JSON.

Validation

• go test ./... -run '^$'
• Focused Go tests for session behavior and generated RPC union behavior passed before commit.

No snapshot updates are included in this PR.

[Copilot]

Pull request overview

This PR updates the Go code generator and the generated Go SDK/RPC types to represent JSON-schema unions as typed, discriminator-driven interfaces (with concrete variant structs and raw fallback variants), and wires the handwritten Go session + tests to use the new Type()-derived event typing and concrete union variants.

Changes:

• Introduces generated JSON (un)marshalling helpers for interface-based unions in new z*encoding.go files for both RPC and session events.
• Refactors generated Go RPC/session types from “flattened optional fields” into discriminated union interfaces with concrete variants (and raw fallback for unknown discriminators).
• Updates Go session logic and E2E/unit tests to use SessionEvent.Type() and type assertions on event.Data / concrete RPC union variants.

Show a summary per file

File	Description
test/scenarios/sessions/streaming/go/main.go	Switches streaming scenario to event.Type() and updated response printing.
go/zsession_encoding.go	Adds generated session-event JSON encoding/decoding and union unmarshalling helpers (including raw fallback for unknown event/data discriminators).
go/session.go	Updates handwritten session code to construct/consume new typed union variants for UI elicitation + permission decisions.
go/session_test.go	Adjusts unit tests for event dispatch and capability updates without relying on a mutable SessionEvent.Type field.
go/session_event_serialization_test.go	Updates serialization tests to validate Type() is derived from Data and raw unknown events round-trip.
go/rpc/zrpc_encoding.go	Adds generated RPC JSON encoding/decoding + union unmarshalling helpers for interface-valued fields.
go/rpc/generated_rpc.go	Updates generated RPC types to use discriminated union interfaces + concrete variants instead of flattened structs.
go/rpc/generated_rpc_union_test.go	Updates/extends unit tests validating JSON union behavior for elicitation schema + values.
go/internal/e2e/testharness/helper.go	Updates E2E helpers to use event.Type() / event.Data type assertions.
go/internal/e2e/*.go	Broad E2E updates for event typing, permissions, attachments, streaming fidelity, and union-shaped payload assertions.

Copilot's findings

• Files reviewed: 28/28 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 39/41 changed files
• Comments generated: 2