Cloud Agent: Tasks API experimental setting and backend (#317206)

* Cloud Agent: Tasks API experimental setting and backend

Introduces a CloudAgentBackend seam with two implementations:

- JobsApiBackend wraps the existing sweagentd Jobs API (default, no behavior change).

- TaskApiBackend implements the new Mission Control Task API and currently uses a StubTaskApiClient pending CAPI routing.

Selection is controlled by the new 'github.copilot.chat.cloudAgentBackend.version' setting ('v1' = Jobs API, 'v2' = Task API). The setting is tagged experimental and defaults to 'v1', preserving existing behavior for legacy users.

* Review comments

* Address PR review feedback

- TaskArtifactPullData: add optional 'number' field separate from db id
- Add ListTaskEventsOptions for event-specific pagination/filters
- Relax sendFollowUp success check (only undefined is failure)
- Drop unused _configurationService field; read config locally in ctor
- Document that cloudAgentBackend.version requires reload