fix: Preserve explicit zero timedelta values in discovery manifests

[hnicke]

Passing explicit timedelta(seconds=0) timedelta values for inactivity timeout, abort timeout, journal retention and idempotency retention was silently dropped - instead, the global defaults were used.
Now, settings the retention times to 0s properly disables the rentention.

Fixes #172

[github-actions]

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[hnicke]

I have read the CLA Document and I hereby sign the CLA

[hnicke]

recheck

[slinkydeveloper]

Nice catch, the timedelta(0) being falsy trap is easy to miss and this is the right fix.

One thing though: workflowCompletionRetention a few lines down still uses the old if handler.workflow_retention check, so a workflow_retention=timedelta(0) would still get dropped. Same bug, just missed it in this pass. Mind switching that one to is not None too while you're here, so all the retention/timeout fields behave the same?

Also would be great to add a small test asserting timedelta(0) ends up as 0 (and not None) in the computed manifest, so this doesn't regress.

[hnicke]

The changeset now also covers the if handler.workflow_retention check and adds regression tests.

[slinkydeveloper]

Thank you for the contribution!