refactor(sync): make session events schema-first

[kitlangton]

Summary

• teach SyncEvent.define(...) to work from native Effect Schema payloads and derive zod internally for runtime/bus compatibility
• migrate Session.Event.Created, Updated, and Deleted to Schema payloads
• migrate MessageV2.Event.Updated, Removed, PartUpdated, and PartRemoved to Schema payloads too

What this finishes

This PR completes the sync-event side of the session/message Effect Schema migration:

• SyncEvent is now Schema-only at the definition layer
• the old mixed z.ZodObject | Effect Schema path is gone
• the temporary phantom _data / _propertiesData fields are gone
• bus payload typing now derives from the stored effectProperties schema directly

Follow-on cleanup included

• remove the last updateSchema(zodObject(...)) interop from session.ts
• reuse def.properties on the bus publish path
• reuse isRecord(...) for sync conversion object checks
• remove an extra session.get(...) read in share-next
• export and reuse Session.Patch in the session projector
• update sync tests to define events with Schema.Struct(...), matching production

Behavior change

This changes the generated SDK type for session.updated patches:

• before: fields in data.info were required and nullable
• after: fields in data.info are optional and nullable

Example:

• before: title: string | null
• after: title?: string | null

This matches the actual patch semantics more closely:

• missing key = unchanged
• key: null = cleared
• key: value = updated

Verification

• bun typecheck
• ./packages/sdk/js/script/build.ts
• push hook also ran repo typecheck successfully