Handle non-resolvable deferred interfaces in dcnm_interface

[dacasti2mx]

Fixes #658

Summary

This change fixes a crash in dcnm_interface during state=overridden when the module processes non-deletable Ethernet interfaces returned by NDFC without a resolvable underlay policy source.

Problem

In the deferred override flow, the module assumes that every non-deletable Ethernet interface has:

underlayPolicies[0]["source"]

That is not always true.

With NDFC 4.2, ISN and External fabrics can return physical Ethernet interfaces with:

• deletable: false
• underlayPolicies: null

When those interfaces enter the deferred path, the module crashes with:

TypeError: 'NoneType' object is not subscriptable

Root Cause

The deferred flow treats all deletable=false Ethernet interfaces as if they were resolvable later via underlayPolicies[].source.

That assumption is invalid for protected or unmanaged Ethernet interfaces in ISN and External fabrics.

Changes

• Added a helper to safely extract a usable underlay policy source
• Added a helper to record non-resolvable deferred interfaces under diff.skipped
• Updated the override flow so that Ethernet interfaces are only added to deferred when:
  • deletable == false, and
  • a valid underlayPolicies[].source exists
• Added a defensive check in the deferred loop to avoid direct indexing of underlayPolicies[0]["source"]
• Added a unit test that covers:
  • state=overridden
  • physical Ethernet
  • deletable=false
  • underlayPolicies=None

Behavior After This Change

For non-deletable Ethernet interfaces without a resolvable policy source, the module now:

• does not crash
• does not add them to deferred
• reports them in diff.skipped
• leaves them untouched

This preserves the existing deferred behavior for interfaces that do have a valid source.

Validation

Manual validation:

• NDFC 4.2
• ISN fabric: remove flow completed successfully
• External fabric: remove flow completed successfully
• Problematic physical Ethernet interfaces were reported under skipped
• No NoneType or subscriptable failure was observed

Unit validation:

• Added coverage for deletable=false plus underlayPolicies=None in test_dcnm_intf.py

[dacasti2mx]

Hi @mikewiebe, when you have time, could you please provide feedback for this PR?
thanks.