Tests unit tests for agent sdk lifecycle (pr #174)

[clintjeff2]

Motivation

• Provide unit coverage for the Agent SDK lifecycle behaviors (start/stop/restart/task execution/heartbeat) that were added without tests.
• Ensure runtime state transitions match the tested SDK contract so callers and health checks observe consistent statuses.

Description

• Add new test suite lib/agent-runtime/__tests__/agent.test.ts covering start(), stop(), restart(), executeTask() behavior, heartbeat timing using fake timers, and the /api/agents/[id]/task 404 case.
• Change Agent.start() to set status to running and Agent.stop() to set status to stopped, and make tasks return the agent to running after completion.
• Make executeTask() throw when invoked on a stopped (or offline) agent.
• Expand the AgentStatus union to include running and stopped, and update the valid-status lists in lib/agents/agent-health-store.ts and lib/agent-registry.ts.
• Update app/api/agents/[id]/task/route.ts to return 404 { ok: false, error: "agent_not_found" } for unknown non-registry agent IDs while preserving runtime creation for generated bot-<n> ids.

Testing

• Ran the new test file with Vitest: npm test -- lib/agent-runtime/__tests__/agent.test.ts and it passed (7 tests).
• Ran the new route test together with the agent tests: npm test -- lib/agent-runtime/__tests__/agent.test.ts __tests__/api/agents/task-rate-limit.test.ts and both test files passed (8 tests total).
• Ran the full test and typecheck flows; the overall test suite and tsc --noEmit surfaced unrelated repo issues (missing optional dev dependencies like lru-cache and @wagmi/connectors) and existing lint failures, which are outside the scope of this change.

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Closes #176

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[clintjeff2]

@leocagli, review and merge.

[clintjeff2]

@leocagli, please review and merge.

[clintjeff2]

@leocagli, please review and merge.