fix: all written artifacts should be saved and garbage collected

[juliev0]

Fixes #13583

Motivation

Two issues:

1. Optional artifacts that were not written were still being passed back through the WorkflowTaskResult, which was causing Artifact GC to try to delete them (and fail to do so)
2. If any non-optional artifact did fail to get saved because it wasn't present, other artifacts would not get saved

Modifications

To address # 1 above, only write to WorkflowTaskResult the artifacts which have been written, whether or not they're marked "Optional". This is what Artifact GC will attempt to delete.

To address # 2 above, don't prematurely return if an artifact fails to be saved.

Verification

e2e test for Artifact GC now includes tests for these cases.
To test # 1 above, see artgc-optional-artifact-not-written.yaml, which includes an Optional artifact that isn't present, which we confirm does not fail ArtifactGC. We also confirm that the Workflow succeeds in this case.

To test # 2 above, see artgc-non-optional-artifact-not-written.yaml , which includes a non-Optional artifact that isn't present, plus another artifact which is present and we confirm does get saved. We also confirm that the Workflow succeeds in this case, since the Artifact that wasn't present was non-optional.

[juliev0]

Hey @agilgur5 - I know you mentioned saving artifacts in parallel would be a good improvement. My thought was that with what I did it's clearly just a "fix" (which could theoretically be patched), and that saving in parallel would be more of a feature. I was also paranoid there could be some new risk of doing it in parallel, that some artifact type may not be able to handle the concurrent saves, although maybe that's not likely. If you feel this is silly, I can revise.

[isubasinghe]

LGTM