feat: add supervision strategy support for delayWith

[He-Pin]

Motivation

Per the stream error handling docs, operators that apply user-provided functions should consult the configured SupervisionStrategy. The delayWith operator computes each element's delay via a user-provided DelayStrategy.nextDelay, but it was called without a try-catch, so any exception failed the stream unconditionally, even under Supervision.Resume/Restart.

This is part of the meta-issue #3110 (add supervisor strategy support to stream operators that accept user functions). One operator per PR.

Modification

• Wrap delayStrategy.nextDelay(element) in Delay.grabAndPull() with a try/catch (NonFatal) that consults the SupervisionStrategy decider:
  • Stop → fail the stage (unchanged fail-fast behavior)
  • Resume → drop the offending element, keep already-buffered elements and the current strategy
  • Restart → recreate the (potentially stateful) delay strategy from the supplier; already-buffered elements keep their computed delays
• delayStrategy became a var so Restart can recreate it.
• Buffer-underflow guard: because a dropped element no longer enqueues, onPush and onTimer now guard their buffer access with a non-empty check. In particular, onTimer could previously hit a NullPointerException (the delay buffer is an unchecked fixed-size buffer) when an overflow handler emptied the buffer while a timer was still armed and a nextDelay exception was then skipped. A dedicated regression test reproduces this (it fails without the guard).
• decider is a lazy val → zero overhead on the happy path.
• Document supervision adherence on delayWith in the Scala/Java DSL scaladoc and the operator reference page (plain delay is unaffected — it uses a fixed internal strategy that never throws).

Result

delayWith now adheres to the SupervisionStrategy attribute with well-defined Resume/Restart semantics and no buffer underflow on the supervised paths.

Tests

• sbt "stream-tests/Test/testOnly org.apache.pekko.stream.scaladsl.FlowDelaySpec" — 26/26 passed, including 6 new tests: Stop, default Stop, Resume (drops element), Restart (recreates the stateful strategy — verified via an instance counter), Resume (keeps the stateful strategy — inverted mirror), and an overflow-handler + throw regression that guards the onTimer underflow fix.
• sbt "stream/mimaReportBinaryIssues" — clean (binary compatible)

References

Refs #3110

This is a clean-room implementation written directly for Apache Pekko.