[Task] Make AgentExecutor take eventqueue.Writer

[yarolegovich]

Passing eventqueue.Queue to Execute and Cancel provides unnecessarily wide API surface to AgentExecutor implementers. This can be changed to eventqueue.Writer.
Because it's a breaking change we can postpone it until 1.0.0 release.

[yarolegovich]

Taking #115 into account where eventqueue gets repurposed for applied task update delivery, there's a suggestion to get rid of queues in AgentExecutor and maybe also rename it to AgentAdapter:

type AgentAdapter interface {
  Execute(ctx context.Context, reqCtx *RequestContext) iter.Seq2[a2a.Event, error]
  Cancel(ctx context.Context, reqCtx *RequestContext) iter.Seq2[a2a.Event, error]
}

The queue removal rationale is to make it more explicit that emitted events first need to be validated and processed (locally where Execute is running) and only after they are written to TaskStore queues come into play.

The renaming rationale is subjective. The new name arguably better describes the purpose of the interface.