[dmt] fix YAML document splitting for block scalars

[diyliv]

Summary

• handle YAML block scalar headers with indentation/chomping indicators when splitting rendered manifests
• keep document separators inside ConfigMap string data intact
• add regression tests for |2 block scalars with embedded ---

Example

A ConfigMap can contain another YAML stream as plain string data. Helm and Kubernetes accept this manifest: the |2 marker starts a YAML literal block scalar with an explicit indentation indicator, so the nested --- is just text inside data.apply-on-startup.yaml.

apiVersion: v1
kind: ConfigMap
metadata:
  name: xxx
data:
  apply-on-startup.yaml: |2
    kind: token
    version: v2
    metadata:
      name: teleport-proxy
    ---
    kind: token
    version: v2
    metadata:
      name: teleport-appservice

Before the fix, DMT only recognized |, |-, >, and >- as multiline scalar markers. It did not recognize |2, so the rendered manifest splitter treated the indented --- as a Kubernetes document separator.

As a result, DMT effectively tried to parse the second half as a standalone manifest:

kind: token
version: v2
metadata:
  name: teleport-appservice

That could fail during manifest unmarshalling with an error like:

manifest ("templates/config.yaml") unmarshal: error converting YAML to JSON: yaml: line 2: mapping values are not allowed in this context

After the fix, DMT recognizes block scalar values by prefix, so |2, |+, |-2, >2, and similar valid YAML forms enter multiline mode. The embedded --- stays inside the ConfigMap string value, and DMT parses the whole ConfigMap as one rendered Kubernetes manifest.