fix(codegen): extend discriminated union detection to support 'type' field

[birdmanmandbir]

Summary

• Extended isDiscriminatedUnion() to detect both method and type field discriminators (was only method)
• Added discriminatorField() helper that returns the discriminator field name
• New genTypeDiscriminatedUnion() generates: struct with Type string + json.RawMessage Data, custom UnmarshalJSON/MarshalJSON, type constants, and typed As*() getter methods
• Covers ThreadItem (14 variants), ResponseItem (11), UserInput (5), EventMsg (75), and 20+ other oneOf types using type as discriminator

Test plan

• go build ./... passes
• go test ./... passes
• No duplicate type definitions across generated files
• Generated unions have correct struct, marshal/unmarshal, constants, and getter methods

[birdmanmandbir]

PR Review: fix(codegen): extend discriminated union detection

Summary

This PR extends the code generator to handle type-discriminated unions (discriminated by a type field with enum values) in addition to the existing method-discriminated unions (discriminated by a method field). This affects types like EventMsg, ThreadItem, ResponseItem, UserInput, AgentMessageContent, and many others.

Changes

• cmd/codegen/main.go: Refactored isDiscriminatedUnion() into discriminatorField() returning the field name (method/type/""). Added genTypeDiscriminatedUnion() that generates a wrapper struct with Type string + Data json.RawMessage, custom JSON marshal/unmarshal, type constants, and As*() getter methods for each variant.
• protocol/types_gen.go: Variant structs moved from here into unions_gen.go (generated inline by the union generator). Net removal of ~1400 lines.
• protocol/unions_gen.go: Now contains the full union types with struct definitions, JSON handling, constants, and accessor methods. Net addition of ~4200 lines.

Verification

• go build ./... — passes
• go vet ./... — passes
• go test ./... — passes
• Schema analysis confirms all type-discriminated unions have the type enum on every variant — no partial coverage issues.
• No union has both method and type discriminators, so the priority check in discriminatorField() is safe.

Findings

No critical issues found.

Minor observations (non-blocking):

1. Untyped string constants for type discriminators — The type-discriminator constants (EventMsgTypeError = "error") are untyped strings, while method-discriminator constants use a typed string (MethodXxx ClientRequestMethod = "method/xxx"). This is fine functionally but means type-discriminator constants don't get compile-time type safety when used with switch statements. Consistent either way is fine for a generated SDK.

2. MarshalJSON fallback — When Data is nil, MarshalJSON serializes only {"type": "..."}. This is reasonable for zero-value structs but means constructing a typed union programmatically requires populating Data manually. Fine for a read-heavy SDK.

3. u.Data = append(u.Data[:0], data...) — Correctly avoids sharing the backing array with the caller's slice. Good.

VERDICT: LGTM

[birdmanmandbir]

Triage Update

Review verdict: LGTM — no critical issues.

CI fix: golangci-lint-action upgraded v6→v9 (v6 pulled lint v1.x built with Go 1.24, incompatible with our Go 1.25.5). Also fixed staticcheck S1039 (unnecessary fmt.Sprintf).

CI status: All checks passing.

Merging.