code-mode: use client cell ids and linear observations

[cconger]

Why

Core already owns stable thread and tool-call identity. A separate idempotency key plus a host-generated cell ID adds protocol state, while retaining every arbitrary observation key grows replay storage for the session lifetime.

What

• Generate compact 16-character CellId values in Core from thread ID and tool-call ID.
• Replace the create idempotency key with caller-provided cell_id: CellId in the wire CreateCellRequest.
• Make repeated creation of the same ID idempotent and reject conflicting request contents.
• Replace the observe idempotency key with generation: ObservationGeneration in the wire ObserveRequest.
• Keep the ObserveOutcome variants from code-mode: expose create and observe operations #29291 unchanged and retain only the latest value per cell for replay.
• Implicitly acknowledge a non-terminal observation when Core starts the next generation.
• Add resume-stability, create-conflict, replay, and eviction coverage.

Host/Core wire sequence

sequenceDiagram
    participant Core
    participant Host
    Core->>Core: derive CellId(thread_id, tool_call_id)
    Core->>Host: CreateCellRequest { cell_id, ... }
    Host-->>Core: same CellId
    Core->>Host: ObserveRequest { cell_id, generation: 0, ... }
    Host-->>Core: ObserveOutcome for generation 0
    opt outcome is ObserveOutcome::Yielded
        Core->>Host: ObserveRequest { cell_id, generation: 1, ... }
        Host->>Host: evict generation 0 outcome
        Host-->>Core: ObserveOutcome for generation 1
    end

Loading

Stack boundary

At this commit an ObserveOutcome::Completed or ObserveOutcome::Terminated value has no successor generation, so its replay payload remains retained until session shutdown. #29401 adds explicit terminal acknowledgement and release.

Validation

• just test -p codex-code-mode
• just test -p codex-code-mode-protocol
• Focused Core tests cover deterministic cell IDs and linear observation generations.

Stack parent: #29310.

[jif-oai]

(found by Codex)
This also runs for ConflictingCreate. In that case this attempt was not admitted, but finish_with_error closes and terminates the original cell that owns this ID. Can we make cleanup ownership-aware so a conflicting retry cannot tear down the existing cell?

[jif-oai]

Missing also reaches this insert..

[jif-oai]

nit: created_cells outlives the actor entry, so retaining the full request here pins every script plus a duplicate enabled-tool catalog for the whole session