feat: async chain end to end

[Theauxm]

Summary

The six internal trains (ManifestManagerTrain, JobDispatcherTrain, JobRunnerTrain, DeadLetterCleanupTrain, MetadataCleanupTrain, InMemoryManifestManagerTrain) drop the now-redundant async keyword on RunInternal. The chain itself returns Task<Either<,>> after the Trax.Core change, so async + await Resolve() was double-wrapping.

No behavior change. Polling services, advisory lock acquisition, and job submitters are untouched.

Test plan

• Unit + Integration + SQLite suites all pass (799 tests: 144 + 634 + 21)
• Postgres integration tests included in 634
• Manifest dependency chain still resolves end-to-end
• pg_try_advisory_xact_lock semantics preserved across the await train.Run(...) boundary
• dotnet build zero warnings
• dotnet csharpier check . passes

Migration

See Trax.Docs migration guide.

[traxsharp]

This PR is included in version 1.28.0

[traxsharp]

This PR is included in version 1.28.0