Support stored screenshot references in computer-use tasks

[danielpeng2]

Description

Client half of computer-use screenshot offload: the server now uploads each screenshot to object storage at ingestion and sends an UpdateTaskMessage swapping the inline bytes in the task message for a StoredScreenshotRef, so screenshots stop traveling base64-inline in every request. This PR advertises the supports_stored_screenshots capability (behind a new StoredScreenshots dogfood-enabled feature flag — the server only swaps for clients that opt in), applies those updates, and keeps screenshot UX working when only a ref exists.

The substantive change is in UpdateTaskMessage handling, which previously assumed updates target a message from the current response stream. The swap targets messages from earlier exchanges, so upsert_message now resolves the owning exchange by message ID (via added_message_ids), tolerates the current stream having added no exchange for the task, and applies the rendered-output upsert to the exchange that actually owns the message — falling back to current-stream behavior only for genuinely new messages. Same-stream flows are unchanged; this is the area most worth review scrutiny.

Elsewhere: UseComputerResult::Success now carries an optional stored ref alongside the action result, ref-only screenshots restore as "no inline image" with the ref retained (the proto task source is what echoes back to the server, so refs must survive round-trips), and the lightbox fetches ref-only screenshots on demand — signed URL from GET /api/v1/agent/conversations/{cid}/screenshots/{uid}, bytes into the AssetCache, error toast on failure. Live sessions keep rendering locally captured bytes and never fetch.

Draft caveats: depends on warpdotdev/warp-proto-apis#367 — Cargo.toml carries a temporary [patch] pointing the proto crate at a local checkout (plus the resulting Cargo.lock delta); both revert when the proto change is published and the git rev is bumped. Server half: warpdotdev/warp-server#16148. Master has not been merged in yet; that plus full presubmit are still owed before this leaves draft.

Linked Issue

• No linked issue — this tracks the screenshot-offload work planned in the linked Warp conversation below.
• No screenshots included — the change has no visible UI beyond the existing lightbox, which is unchanged when bytes are present.

Testing

Added 4 unit tests in task_tests.rs: same-stream update regression, cross-exchange stored-ref swap on a UseComputer result (including shared-session input replacement), an update arriving when the current stream added no exchange, and the new-message ExchangeNotFound error case. Ran: cargo nextest run -p warp -E 'test(test_upsert_message)' (4/4 pass), cargo check -p ai -p warp_features, cargo check -p warp --lib, ./script/format, and cargo clippy -p warp -p ai --lib --tests -- -D warnings — all pass. Full presubmit not run yet (draft).

• Not manually tested yet — end-to-end manual testing needs the server flag on staging or a local server with the staging bucket applied; planned before marking ready.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-NONE

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Warp conversation · Plan: Offload computer-use screenshots to GCS, keep references in the MAA task

[danielpeng2]

• Support stored screenshot references in computer-use tasks #15587 👈 (View in Graphite)
• master

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