fix(a2a): include assistant turns in conversation history

[justintime4tea]

A follow-up message on an existing contextId was given the prior user turns but never the agent's own replies, so it answered as if it had never spoken.

get_history_for_context rebuilt history from Task::history alone. The upstream request handler writes that field once, with the prompt, and never appends to it; the agent's reply is streamed out as artifacts instead, leaving the Role::Agent arm of convert_a2a_msg_to_aura unreachable. The same call also passed include_artifacts: Some(false), so the reply was never fetched, and sorted the context's tasks newest-first, which would have replayed a multi-task exchange backwards.

History now rebuilds the assistant turn from the artifacts the client saw: the streamed "response" chunks concatenated with no separator, falling back to "final" for a reply that only arrived as a final response. Tool-call, reasoning, and scratchpad artifacts are skipped. Tasks replay oldest-first, and a task already carrying a ROLE_AGENT message keeps it rather than gaining a second one. The two artifact ids are now module consts shared with the sites that emit them, so reader and writer cannot drift apart.

A2A clients still see only the prompt in task.history. The upstream handler owns every task write and carries a stale in-memory task across events, so writing the agent turn back would either race it or have to go through SharedTaskStore, which conflicts with the Redis store's terminal-immutability rule. Reconstructing on read works with both stores and touches no write path.

test_sequential_tasks_share_the_assistant_turn covers the half that test_sequential_tasks_share_context cannot. It routes a per-run secret in through a forwarded header that the agent reports, then sends the follow-up without that header so the tool cannot surface the value again, and asserts the follow-up made no tool call at all, leaving the prior assistant turn as the only possible source.

Fixes: #368

[greptile-apps]

Greptile Summary

This PR reconstructs prior assistant turns from persisted A2A response artifacts so follow-up tasks receive complete conversation history.

• Shares artifact identifiers between the stream writer and history reader.
• Fetches artifacts and replays prior tasks oldest-first.
• Avoids duplicating assistant turns already recorded in task history.
• Adds unit and integration coverage for streamed chunks, final-only replies, ordering, and sequential context reuse.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete blocking or independently actionable non-blocking defects identified.

The changed history path consistently reads the same artifact identifiers emitted by the executor, prefers streamed response text with a final-only fallback, prevents duplicate recorded assistant turns, and is covered across reconstruction and sequential-context scenarios.

Important Files Changed

Filename	Overview
crates/aura-web-server/src/a2a/agent_executor.rs	Reconstructs assistant history from response/final artifacts, requests artifacts during task listing, and reorders prior exchanges oldest-first with focused unit coverage.
crates/aura-web-server/tests/a2a_test.rs	Adds an end-to-end sequential-task test proving a follow-up can recall a value present only in the prior assistant reply, plus reusable A2A test helpers.

Sequence Diagram

sequenceDiagram
    participant Client
    participant Handler as A2A Handler
    participant Store as Task Store
    participant Executor
    participant Agent
    Client->>Handler: Send first user message
    Handler->>Store: Persist task prompt
    Handler->>Executor: Execute task
    Executor->>Agent: Stream prompt with prior history
    Agent-->>Executor: Response chunks and final response
    Executor-->>Handler: Response/final artifacts
    Handler->>Store: Persist artifacts
    Client->>Handler: Send follow-up with contextId
    Handler->>Executor: Execute follow-up
    Executor->>Store: List context tasks with artifacts
    Executor->>Executor: Rebuild user and assistant turns
    Executor->>Agent: Stream follow-up with complete history

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[Shearerbeard]

Approving for now but i think after we flesh out park/reify #271 and get closer to task attach/detach this will have the same generic home + backend for all agent initializations - not just specific A2A

[greptile-apps]

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