feat: change v4 payload topic to v3 format, add topicBytes field

[xmtp-coder-agent]

Resolves #101

Summary

• buildV4SendRequest now sets Topic to V3 string format (TopicToLegacy) instead of V4 base64 (TopicToBase64)
• New TopicBytes field on SendRequest carries the V4 binary topic as base64
• APNS: includes topicBytesB64 custom field when present
• FCM: includes topicBytesB64 in data map and APNS custom data when present
• HTTP: topicBytesB64 included in JSON via MarshalJSON (omitted when empty)
• All existing V3 delivery paths unchanged — topicBytesB64 only appears for V4 payloads

Files Changed

File	Change
pkg/interfaces/interfaces.go	Add TopicBytes to SendRequest, topicBytesB64 to JSON marshaling
pkg/xmtp/v4_listener.go	buildV4SendRequest uses TopicToLegacy for Topic, TopicToBase64 for TopicBytes
pkg/delivery/apns.go	Conditionally add topicBytesB64 to APNS notification payload
pkg/delivery/fcm.go	Conditionally add topicBytesB64 to FCM data and APNS custom data

Test plan

• pkg/interfaces — new tests for TopicBytes JSON marshaling (included + omitted when empty)
• pkg/delivery — new V4 tests for APNS and FCM topicBytesB64 field
• pkg/xmtp — updated V4 listener tests assert V3 topic format + TopicBytes field
• go build ./... — compiles cleanly
• go vet ./... — no issues
• Non-DB tests pass (pkg/interfaces, pkg/delivery, pkg/topics)
• DB-dependent tests (pkg/xmtp, pkg/db) require PostgreSQL — verified logic changes only

🤖 Generated with Claude Code

Note

Add topicBytesB64 field to V4 push notification payloads and set V4 topic to legacy format

• V4 SendRequest now carries both a legacy string Topic and a new TopicBytesB64 field containing the base64-encoded topic bytes, set in v4_listener.go.
• topicBytesB64 is conditionally included in APNS custom payloads and FCM Data/APNS.CustomData when the field is non-empty.
• SendRequest.MarshalJSON emits topicBytesB64 in the JSON wire format when set, via the interfaces.go custom marshaler.
• Behavioral Change: V4 Topic is now set to the legacy string representation instead of raw topic bytes, which affects downstream consumers reading the Topic field.

^{Macroscope summarized 5507525.}