[Think] Add client projection hooks for server-only message context

[c-w-xiaohei]

Problem

Think Session is the canonical history used for model execution, tools, compaction, and recovery. Applications may need to persist server-owned, model-visible context with a turn, such as retrieval snapshots, system reminders, or internal tool details, without exposing it to browser clients.

Today, Think uses the same this.messages view for model assembly and client synchronization. Persisting internal parts therefore risks exposing them through broadcasts, /get-messages, reconnects, rollback, or resumable replay. beforeTurn is too late to persist context, while Agent lifecycle hooks do not cover every client output path. This forces applications to either split conversation state into sidecars or replace the built-in browser transport.

Proposal

Please consider stable hooks such as:

prepareMessageForSession(message, context): SessionMessage
projectMessagesForClient(messages, context): UIMessage[]
projectStreamChunkForClient(chunk, context): UIMessageChunk | null

The exact API may differ, but the important guarantees are:

• server-only fields are created or validated before Session persistence;
• model execution, compaction, and recovery use the complete canonical history;
• every client-facing history, broadcast, reconnect, rollback, and replay path uses the client projection;
• stream projection preserves complete tool-call lifecycles and terminal events;
• clients cannot mark their own content as trusted/internal.

Why

A single canonical Session transcript should be able to support different audiences without requiring application-level transcript joins or a second chat protocol. This would provide one extensible boundary for retrieval snapshots, reminders, private server tools, and future model-only context.

Observed with @cloudflare/think@0.13.0 and agents@0.17.3.

(co-authored by gpt-5.6-sol)