refactor(core): migrate MessageV2 message DTOs (User/Assistant/Part/Info/WithParts) to Effect Schema

[kitlangton]

Summary

Fourth piecemeal slice of the session-domain Effect Schema migration, stacked on #23756.

• Migrates User, Assistant, Part discriminated union, Info, and WithParts to Effect Schema.
• Replaces the cast-based Zod Part definition with a proper Schema.Union that annotates `discriminator: "type"` + `identifier: "Part"`.
• Assistant.error keeps a Zod-side discriminated union via ZodOverride because NamedError.create(...) errors are still Zod (error migration is intentionally out of scope for this PR).
• Event payloads (`SyncEvent.define`, `BusEvent.define`) still take Zod inputs and now reference the derived `.zod` on each schema.
• Updates callers that were reaching into the Zod API directly:
  • `session.ts` (`MessageV2.Assistant.shape.error` → `.zod.shape.error`)
  • `server/routes/instance/session.ts` (`MessageV2.{Info,Assistant,Part,WithParts}` → `.zod`)
  • `session/prompt.ts` (`.safeParse` → `.zod.safeParse`)
  • `cli/cmd/import.ts` (`.parse` → `.zod.parse`)
  • `test/session/structured-output.test.ts` (`.safeParse` → `.zod.safeParse`)

Acceptance check

• `bun typecheck` (package + repo root) clean.
• `bun run test` → 2010 pass / 0 fail.
• `bun dev generate` byte-identical to committed `packages/sdk/openapi.json`.
• `packages/sdk/js/src/v2/gen/types.gen.ts` unchanged after `bun script/build.ts`.

Follow-up

Remaining Zod in `message-v2.ts`:

• `NamedError.create(...)` error classes (`OutputLengthError`, `AbortedError`, `StructuredOutputError`, `AuthError`, `APIError`, `ContextOverflowError`). These ripple across `retry.ts`, `processor.ts`, `prompt.ts` `isInstance`/`toObject` callers, plus other files. Best approached as its own PR using a bridge on top of `Schema.TaggedErrorClass`.
• Internal `Cursor`. Not worth a dedicated PR; can ride along with a future cleanup.
• `SyncEvent.define` / `BusEvent.define` payloads remain `z.object` until those APIs move to `Schema`.