lint: enable protogetter linter (#7336)

Enable protogetter in golangci config and update all protobuf field
access to use getter methods instead of direct field access.
Getter methods provide safer nil pointer handling and return
appropriate default values, following protobuf best practices.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

.golangci.yml

@@ -8,6 +8,7 @@ linters:
     - ineffassign
     - intrange
     - nolintlint
+    - protogetter
     - staticcheck
     - unconvert
     - unused