Replace `nil, nil` returns with sentinel errors in status helpers

Summary

Several functions in the status packages return nil, nil (no result and no error) to indicate "not found" or "not applicable" conditions. Each has a //nolint:nilnil comment noting that a sentinel error would be more ergonomic. This makes callers harder to reason about since they must check both return values for nil.

Locations

pkg/pod/status.go:

• Line 569: return nil, nil //nolint:nilnil // would be more ergonomic to return a sentinel error
• Line 584: return nil, nil //nolint:nilnil // would be more ergonomic to return a sentinel error

pkg/status/status.go:

• Line 42: return nil, nil //nolint:nilnil // would be more ergonomic to return a sentinel error
• Line 60: return nil, nil //nolint:nilnil // would be more ergonomic to return a sentinel error

What to do

1. Define sentinel errors in the appropriate package, e.g.:

   var ErrNotFound = errors.New("resource not found")
   

2. Replace return nil, nil with return nil, ErrNotFound (or a more descriptive sentinel)
3. Update callers to use errors.Is(err, ErrNotFound) instead of checking for nil result
4. Remove the //nolint:nilnil comments

How to find callers

# Find functions that return nil, nil
rg "return nil, nil" pkg/pod/status.go pkg/status/status.go

# Find callers of those functions to update
# (use your IDE or grep for the function names)

How to verify

• go build ./... passes
• go test ./pkg/pod/... ./pkg/status/... passes
• go vet ./... passes
• The nilnil linter no longer needs to be suppressed for these locations

/kind cleanup