Add tests for backend_type propagation across agent lifecycle

[geoffjay]

Summary

Add test coverage verifying that backend_type is correctly set when agents are
spawned, restarted, and reconciled. The existing test harness in
crates/orchestrator/tests/ already constructs ApiState with
backend_type: BackendType::Tmux but no tests assert the value flows through
to the agent record.

Context

Existing tests in conversation_persistence.rs and system_agent_http.rs
create ApiState with a hardcoded BackendType::Tmux. After #1235 and #1236
land, the AgentManager itself will be responsible for setting the backend
type on agents. Tests should verify:

1. A spawned agent's backend_type matches the backend in use
2. A restarted agent's backend_type is updated if the backend changed
3. The reconciliation backfill (Backfill backend_type for existing agents during startup reconciliation #1237) corrects stale values
4. The API response (AgentResponse) carries the correct backend_type

Acceptance Criteria

• Add a unit test in crates/orchestrator/ (or integration test in
  crates/orchestrator/tests/) that spawns an agent via the API and asserts
  response.backend_type == Some("tmux") (or whichever test backend is used)
• Add a test that verifies backend_type is persisted to storage correctly
  (read back from DB after spawn)
• Add a test that verifies AgentResponse::from(agent) preserves the
  backend_type field
• If feasible, add a test with a mock backend that returns a non-tmux
  backend_name() to verify the value is not hardcoded
• cargo test passes

Key Files

• crates/orchestrator/tests/system_agent_http.rs -- existing test harness
  with ApiState construction (line 114)
• crates/orchestrator/tests/conversation_persistence.rs -- another test
  harness example (line 118)
• crates/orchestrator/src/types.rs -- AgentResponse::from(Agent) impl
• crates/orchestrator/src/manager.rs -- spawn_agent(), restart_agent()

Blocked By

• Set correct backend_type in AgentManager spawn_agent and restart_agent #1236 (tests must verify the fixed behavior, not the broken default)

Stack Base

Stack on: feature/autonomous-pipeline
Blocked by: #1236
Parallel: no ordering constraint with #1237 or #1238 (test files only)

[geoffjay]

✅ Triage Complete

Field	Value
Type	enhancement
Complexity	medium
Milestone	v0.3.0 — Stabilization & Code Quality
Labels	enhancement, needs-tests, complexity:medium

Notes:

• No duplicates found. Issues Add backend_name() method to ExecutionBackend trait #1235–Clean up MCP and CLI backend_type display fallbacks #1238 are sibling implementation issues in the same backend_type fix series; Add tests for backend_type propagation across agent lifecycle #1239 is distinctly scoped to test coverage.
• The bug label was replaced with enhancement — the bug itself is tracked in Set correct backend_type in AgentManager spawn_agent and restart_agent #1236; this issue adds verification tests.
• Complexity medium: four test cases (spawn assertion, storage persistence, AgentResponse::from conversion, and an optional mock-backend case). All work lives within crates/orchestrator/tests/ with the existing ApiState harness. A mock ExecutionBackend may be needed for the fourth criterion, adding modest design work.
• Milestone v0.3.0: these tests verify bug fixes that belong to the stabilization cycle and complement the code quality goal of that milestone.
• Blocked by Set correct backend_type in AgentManager spawn_agent and restart_agent #1236 — tests should not land before the fix they verify.
• Acceptance criteria are fully specified; no clarifying questions needed.