feat(plugin): add session title hook and request options bag - #47663

Summary

First step of splitting session request hooks by request type (per discussion with dax: one hook per kind of LLM request, same shape). Follows #47214 / #47221.

Shared request shape

export type SessionRequestOptions = Types.DeepMutable<GenerationOptionsFields> & Record<string, unknown>

export interface SessionRequest {
  readonly sessionID: Session.ID
  readonly model: Model.Ref
  system: Array<SystemPart>
  messages: Array<Message>
  options: SessionRequestOptions
}

export interface SessionContext extends SessionRequest {
  readonly agent: Agent.ID
  tools: Record<string, { description: string; input: JsonSchema.JsonSchema }>
}

export interface SessionTitle extends SessionRequest {}

options replaces generation + providerOptions. Typed keys (maxTokens, temperature, …) are the protocol-neutral generation settings; any other key (reasoningEffort, serviceTier, …) is passed to the protocol as a provider option. Core partitions by GenerationOptions.fields when building the LLMRequest; the ai package is unchanged.

New session.title hook. Title generation is not an agent conversation, so the event carries no agent or tools. A configured title agent still supplies the default system prompt and model as before — the hook just doesn't surface it as an agent.

SessionModelRequest has one entry per flow. primary, compaction, generate, title. Each entry runs the hook that shapes its flow and sets the request kind; callers no longer pass kind, contextHooks, or contextAgentID, and the input is flat. The compaction entry owns the identity split (context hook sees the session agent, request hooks see compaction) until it gets its own hook. Single-use helpers in model-request.ts are inlined into the shared lowering step.

model stays read-only for now; mutable model needs a two-stage transcript render and will be its own PR.

Unchanged: context still fires for compaction and Session.generate in this PR; their own hooks come next, at which point the built-in system-prompt plugin registers on compaction too for cache-prefix reuse. kind on model.request/http.*/retry is unchanged.

Tests

• New title test: title hook fires (system/messages/options mutable, options split into generation + providerOptions on the request), context does not.
• HTTP-hook kind test now drives each entry directly.
• Existing fixtures in core, server, sdk updated to options / the new entries.

Typecheck clean in plugin, core, server, sdk.