fix(stream): log supervision Resume/Restart in MapAsync operators

[He-Pin]

Summary

• Adds error logging when supervision strategy returns Resume or Restart for failed elements in MapAsync, MapAsyncUnordered, and MapAsyncPartitioned
• Respects the stage-errors-default-log-level configuration and per-stage LogLevels.onFailure attribute from Add fine-grained stream error logging control #2805
• Logging placed to avoid double-logging (uses Holder's directive cache to ensure exactly one log per failure)

Changes

• MapAsync: Added StageLogging mixin, logSupervisionFailure helper, logging in pushNextIfPossible and sync exception handler
• MapAsyncUnordered: Added StageLogging mixin, logSupervisionFailure helper, logging in futureCompleted and sync exception handler
• MapAsyncPartitioned: Added StageLogging mixin, logSupervisionFailure helper, logging in both ordered and unordered pushNextIfPossible and sync exception handler

Motivation

MapAsync, MapAsyncUnordered, and MapAsyncPartitioned silently skip failed elements when the supervision strategy returns Resume or Restart. This makes production debugging difficult — users cannot see which elements were dropped or what exceptions occurred.

Modification

• Mix in StageLogging trait for all three operators
• Add logSupervisionFailure helper that checks LogLevels.onFailure attribute, falls back to LogLevels.defaultErrorLevel(system), and logs at the configured level
• For MapAsync and MapAsyncPartitioned, logging is placed in pushNextIfPossible (not in futureCB or the fast path) to avoid double-logging since the Holder caches the supervision directive
• For MapAsyncUnordered, logging is placed in futureCompleted (no buffer re-processing)
• Sync exception handlers log directly since they bypass the Holder caching

Result

Supervision Resume/Restart decisions now produce log entries with exception details, enabling users to diagnose dropped elements. Log level can be controlled via pekko.stream.materializer.stage-errors-default-log-level config or per-stage ActorAttributes.logLevels.

Tests

• FlowMapAsyncSpec: 3 new tests (async future failure logs, sync throw logs, log level Off disables logging)
• FlowMapAsyncUnorderedSpec: 3 new tests (same pattern)
• FlowMapAsyncPartitionedSpec: 3 new tests (ordered, unordered, log level Off)
• All 74 existing + new tests pass

References

Fixes #3103

[pjfanning]

lgtm