Task retries

A2A Mesh exposes TaskRetryScheduler helpers for retry planning, capped backoff, due checks, success, and dead-letter transitions.

This is an in-process contract for durable task engines. Queue adapters can persist TaskRetryPlan records in Postgres, Redis, BullMQ, Temporal, or another scheduler while keeping the same state and delay semantics.

Helpers

Helper	Purpose
createTaskRetryPlan	Creates a queued retry plan.
markTaskAttemptStarted	Moves a plan to running and increments attempts.
markTaskAttemptFailed	Reschedules the plan or marks it dead-lettered.
markTaskAttemptSucceeded	Marks the plan succeeded.
isTaskRetryDue	Checks whether queued work is ready to run.
calculateRetryDelayMs	Computes capped exponential backoff.