[Task] Provide a generic PullEventQueue

[yarolegovich]

Provide a generic "pull-based" queue in the eventqueue package and migrate examples/clustermode to it.

package eventqueue

type PullCursor any

type PullResponse struct {
  Messages []*eventqueue.Message
  Cursor   PullCursor
}

type Puller interface {
  Pull(ctx context.Context, taskID a2a.TaskID, cusor PullCursor) (*PullResponse, error)
  Close(ctx context.Context) error
}

type PullerProvider func(ctx context.Context, taskID a2a.TaskID) (Puller, error)

func NewStaticPullerProvider(es Puller) PullerProvider { ... }

type PullConfig struct {
    PollInterval      time.Duration // default 30s
    InactivityTimeout time.Duration // default 5m, 0 disables
    AccessCheck       func(context.Context, *a2a.Task) error // optional
    OnInactivity      func(context.Context, Puller, a2a.TaskID) error // optional
}

func NewPullQueueManager(pp PullerProvider, cfg PullConfig) eventqueue.Manager { ... }

NewPullQueueManager should create an inner in-memory manager internally. CreateWriter always delegates to the in-memory manage inner. CreateReader delegates to inner unless the call context indicates a resubscribe (a2asrv.CallContextFrom(ctx).Method()), otherwise a pullReader is returned which:

• Calls Provider to get a per-call Puller.
• Spawns a polling goroutine that calls Pill every PollInterval, pushes events through a channel, and stops on taskupdate.IsFinal.
• First Read emits the Snapshot task as the initial event (matches current emittedSnapshot logic).
• Subsequent Reads either drain the channel, return eventqueue.ErrQueueClosed on final state, return ctx.Err() on
  cancellation, or return inactivity error after InactivityTimeout (with optional OnInactivity callback to refresh state).
• Destroy delegates to inner. The pull-side reader cleans itself up via Close on the source.

[nahapetyan-serob]

#354 this pr provides a generic pull event queue
will open a separate pr for migrating examples/clustermode to it

[nahapetyan-serob]

migrated in #363