Optimize exit code handling by relying on scheduler status for successful executions

[jorgee]

close #6445
close #6481

Some cloud provider implementations got exit code from .exitcode files other get from the API and fallback to .exitcode when exitcode was null or 0.
This PR unifies exit code behaviour in all cloud providers.

1- Try to get exitcode form the API
2. If exitcode is missing (null) fallback to read .exitcode

Note: not applied to grid executors

[netlify]

✅ Deploy Preview for nextflow-docs-staging canceled.

Name	Link
🔨 Latest commit	a2114e9
🔍 Latest deploy log	https://app.netlify.com/projects/nextflow-docs-staging/deploys/68ef94ccf3592c0008c0b24a

[jorgee]

@bentsherman @pditommaso the google batch task handler is directly getting the exit code form the file (#6481). This PR also removes the fallback to .exitcode when the exit code is 0. It is not a big PR but I was wondering if you prefer to split it in two PRs to facilitate backport to stable versions. The fix for #6481 is just the first commit ea1aa48

[pditommaso]

This looks good, but i'd keep post 25.10

[bentsherman]

@jorgee we talked and agreed to include the google and k8s fixes in 25.10 and merge the rest of this PR in the next edge release