[dmt] fix mount points rules

[diyliv]

Summary

• Built-in exclusions for system paths: /sys, /dev, /proc are now ignored by both container and templates mount-points rules. CSI drivers commonly mount these host paths at runtime without declaring them in Helm templates.
• Add files: support to templates rule: mountPointsFile struct now parses files: alongside dirs:, matching the container rule behaviour.
• Fix filepath.Walk error handling: return err changed to return nil + error reporting — a single permission error no longer stops the entire tree walk, which would skip all mount-points.yaml files below the problematic directory.
• Add error logging to container rule: collectMountPointsDirs now logs walk/read/parse errors via log.Warn instead of silently swallowing them.

Context

The csi-netapp module CI was failing with many false warnings from the templates rule:

mount-points.yaml references dir "/dev" which is not used as a mountPath in any pod controller
mount-points.yaml references dir "/etc/iscsi" which is not used as a mountPath in any pod controller
mount-points.yaml references dir "/host" which is not used as a mountPath in any pod controller
...

These are host system paths that the CSI driver accesses at runtime — they are correct but absent from Helm templates. The /sys path was explicitly requested to be excluded. The files: entries in mount-points.yaml were silently ignored by the templates rule. And the container rule swallowed all read/parse errors, making bugs invisible.

Example

Before (CI output for csi-netapp):

mount-points (#templates): mount-points.yaml references dir "/dev" which is not used as a mountPath in any pod controller
mount-points (#templates): mount-points.yaml references dir "/etc/iscsi" which is not used as a mountPath in any pod controller
mount-points (#templates): mount-points.yaml references dir "/host" which is not used as a mountPath in any pod controller

After: /dev is skipped as a built-in system path. Remaining module-specific paths (/host, /etc/iscsi, /etc/multipath, etc.) can be excluded via module config:

templates:
  excludeRules:
    mount-points:
      - /host
      - /etc/multipath
      - /etc/iscsi
      - /run/iscsid
      - /run/multipathd
      - /var/lib/iscsi
      - /var/lib/kubelet
      - /var/lib/trident/tracking
      - /certs
      - /csi