Step Should Retry support

[devhawk]

Adds a shouldRetry predicate to steps, allowing callers to short-circuit retries on specific exception types rather than always exhausting maxAttempts.

Two ways to specify the predicate:

• Annotation (@Step): provide a Class<? extends StepShouldRetry> — the class is instantiated via reflection at runtime. StepShouldRetry is a functional interface w/ boolean shouldRetry(Throwable) method.

@Step(maxAttempts = 5, shouldRetry = MyRetryPolicy.class)
public String myStep(String input) { ... }

• Programmatic (StepOptions): pass a Predicate directly, suitable for inline steps or lambdas.

dbos.runStep(
  () -> { ... },
  new StepOptions("myStep")
    .withMaxAttempts(5)
    .withShouldRetry(e -> !(e instanceof FatalException)));

When shouldRetry returns false, the step aborts immediately and rethrows the original exception. When it returns true (or is not set), normal retry-with-backoff behavior applies.

Implementation notes

• StepShouldRetry.None is a sentinel inner class used as the annotation default, since Java annotations cannot use null as an element default value.
• StepShouldRetry implementers must have a public no-arg constructor

fixes #413