[Feat]: Add task listing endpoint for non-streamed/async A2A agents

### Is your feature request related to a problem? Please describe.

Currently, agents that do not support streaming have no way for clients to poll, list, or query for completed tasks. The inspector only reacts to streaming events, so completed work for non-streamed agents is not visible unless the agent emits an explicit completion message. This presents problems for users working with asynchronous/long-running A2A agents as the UI offers no fallback or visibility into finished tasks outside of streaming responses.

### Describe the solution you'd like

Introduce a 'list tasks' endpoint in the backend and UI that queries the connected A2A agent according to spec (if supported). This API should allow users to see in-progress, completed, and failed tasks regardless of whether the agent streams events or emits completion notifications. Ideally, support polling for individual task status as well as a listing endpoint that shows all agent tasks for the current session/context.

### Describe alternatives you've considered

- Require all agents to emit a completion event, even if not streaming (may not be feasible or spec-compliant).
- Extend frontend polling logic to periodically check for completed tasks if no stream is received.
- Rely solely on agent-side adaptors to ensure streaming compatibility, which isn't practical for all A2A implementations.

### Additional context

The current implementation only displays task progress and output when events are streamed from the agent. Agents that only provide async task execution (without event streaming) are not surfaced in the UI, making inspector usage frustrating for non-streamed agents. This feature would comply with the flexibility described in the A2A spec and make the inspector more robust for a wider set of agent implementations.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The issue describes adding a 'list tasks' endpoint for non-streamed A2A agents. Start by examining the backend API structure, likely in a directory like `src/backend/` or `src/api/`. Look for existing agent connection and task handling logic to understand the data model. Check the A2A spec for task querying requirements. 'Done' means implementing the endpoint and ensuring the UI can call it to display tasks.

Scritto dal modello di indicizzazione a partire dal testo della issue.