test(orchestrator): end-to-end integration tests for conversation persistence

[geoffjay]

Adds 26 integration tests in tests/conversation_persistence.rs covering the storage layer (cursor filtering, ordering, session isolation, all event types) and REST API (GET/DELETE conversation endpoints, pagination, 404 paths). Also implements the missing DELETE /agents/{id}/conversation endpoint (204 No Content) needed by the acceptance criteria.

Closes #1165

[geoffjay]

This change is part of the following stack:

• test(orchestrator): end-to-end integration tests for conversation persistence #1174 ◀

_{Change managed by git-spice.}

[geoffjay]

Review: test(orchestrator): end-to-end integration tests for conversation persistence

Verdict: Approved. The DELETE endpoint implementation is clean and the 26 tests are thorough. All concerns are either informational or tied to parent PR fixes that must land before this merges.

---

✅ DELETE /agents/{id}/conversation — correct

async fn delete_agent_conversation(
    State(state): State<ApiState>,
    Path(id): Path<Uuid>,
) -> Result<impl IntoResponse, ApiError> {
    state.manager.get_agent(&id).await?.ok_or(ApiError::NotFound)?;
    state.manager.agent_storage()
        .delete_conversation_events_for_agent(id).await
        .map_err(ApiError::Internal)?;
    Ok(StatusCode::NO_CONTENT)
}

• Agent existence check ✅
• Delegates to existing idempotent storage method ✅
• 204 No Content ✅
• Route combined on the existing /conversation path with correct Axum .delete() syntax ✅

---

✅ Test structure — solid throughout

• NullBackend correctly implements all ExecutionBackend trait methods — tests compile without external dependencies ✅
• Each test constructs its own TempDir + fresh migrated database — no shared state between tests ✅
• build_app() drives the full Axum router via tower::ServiceExt::oneshot — exercises real routing, real serialization, real storage ✅
• urlencoding correctly handles RFC 3339 timestamps in query strings (: needs encoding) ✅

Coverage breakdown:

Area	Tests
since / until cursor filtering	3 (individual + combined window)
Ordering guarantee (ASC by created_at)	1
Session isolation	1
Cross-agent delete isolation (storage)	1
All 8 event-type variants round-trip	1
JSON metadata round-trip	1
GET /conversation (basic, limit, event_type, session, after, before, 404, empty)	8
GET /conversation/summary (counts + 404)	2
GET /conversation/{event_id} (found, wrong-agent 404, not-found 404)	3
DELETE /conversation (clears, 404, cross-agent isolation, idempotent)	4
count matches list length	1

---

Stack note — deeply blocked

This PR is stacked on issue-1162, which depends on issue-1161 (PR #1170, needs-rework) → issue-1159 (PR #1167, needs-rework). None of the parent PRs can merge yet. No action needed on this PR until the parent chain is repaired.

---

🟡 Session filter tests don't catch the pagination bug flagged in #1170

test_storage_session_isolation has a comment that explicitly documents the in-memory approach:

// Filter in application layer as the API does.
let session_1: Vec<_> = all.iter().filter(|e| e.session_number == 1).collect();

test_api_get_conversation_session_filter inserts 5 total events and uses the default limit (100), so all 5 are fetched before the in-memory retain. The test passes — but a variant with a small limit would fail under the current implementation. Once PR #1170 is fixed to push the session filter to the DB, add:

// After #1170 fix: session filter works correctly when limit < total events
async fn test_api_get_conversation_session_filter_under_limit() {
    // insert 50 session-0 events + 3 session-1 events
    // request ?session=1&limit=2 → should return 2 with has_more=true
}

This test would currently fail; record as a post-fix TODO.

---

🟡 Minor scope note — DELETE handler belongs in #1170

delete_agent_conversation and its route registration are production code added in a test PR. They're correct and the co-location with the DELETE tests is pragmatic, but they would more naturally live alongside the other conversation endpoints in PR #1170.