feat: provide generic pull event queue

[nahapetyan-serob]

This PR provides a generic "pull-based" queue in the eventqueue package

NewPullQueueManager creates an inner in-memory manager internally. CreateWriter always delegates to the in-memory manage inner. CreateReader delegates to inner if PullConfig.UseInMemory returns true, otherwise a pullReader is returned which:

Calls Provider to get a per-call Puller.
Spawns a polling goroutine that calls Pull every PollInterval, pushes events through a channel, and stops on taskupdate.IsFinal.
First Read emits the Snapshot task as the initial event.
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.

[gemini-code-assist]

Code Review

This pull request introduces a pull-based event queue implementation, including a new pullQueueManager and pullReader to support polling events from a Puller, handling inactivity timeouts, and performing access checks. The review feedback highlights several critical issues: NewStaticPullerProvider fails to make Close a no-op as documented, which can prematurely close shared pullers; the channel-based closeSignal in pullReader is prone to deadlocks and should be replaced with context-based cancellation; a bug in the inactivity timeout logic returns ErrInactivityTimeout even when a new task is successfully retrieved, terminating the subscriber loop; and newMessage can be simplified to accept *a2a.Task directly to eliminate redundant type assertions.

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