Even if the exit code is 0. Missing declared outputs will be detected by Bazel later and fail the build, so avoid uploading this false positive cache entry.

Wrap buildUploadManifest inside a `Single.fromCallable` since there are many IOs (both the check we add in this PR and stats already there). If `--experimental_remote_cache_async` is set, these IOs will now be executed in a background thread.

Fixes bazelbuild#14543.

Closes bazelbuild#15016.

PiperOrigin-RevId: 434448255
(cherry picked from commit 5b54588)

An AC entry that misses declared outputs of an action is invalid because, if Bazel accepts this from remote cache, it will detect the missing declared outputs error at a later stage and fail the build.

This PR adds a check for mandatory outputs when downloading outputs from remote cache. If a mandatory output is missing from AC entry, Bazel will ignore the cache entry so build can continue.

Also fixes an issue introduced by bazelbuild#15016 that tests result are not uploaded to remote cache. Test spawns declare all the possible outputs but they usually don't generate them all. This change fixes that by only check for mandatory outputs via `spawn.isMandatoryOutput()`.

Fixes bazelbuild#14543.

Closes bazelbuild#15051.

PiperOrigin-RevId: 435307260
(cherry picked from commit a151116)