fix: render minLength, maxLength, and pattern constraints

[thc1006]

Summary

The built-in markdown.tmpl, frontmatter.tmpl, and adoc.tmpl
templates render several OpenAPI schema constraints, including enum,
format, default, minimum, maximum, and
x-kubernetes-validations.

However, they omit minLength, maxLength, and pattern from the
generated documentation, even though these values are already available
in each field's apiextensions.JSONSchemaProps.

As a result, the generated reference does not fully describe the API
server's validation contract.

This PR renders all three string constraints in every built-in output
template.

Changes

• Update markdown.tmpl and frontmatter.tmpl to render Min Length,
  Max Length, and Pattern, and include these fields in the spacer guard.

• Update adoc.tmpl to render the same constraints.

• Escape the pattern per output format so a valid pattern is shown faithfully:

  • The HTML templates (markdown.tmpl, frontmatter.tmpl) pass the pattern
    through the built-in html function. A pattern may legally contain < and
    > (Go named capture groups (?P<name>...) / (?<name>...)) or &, which
    an HTML renderer would otherwise misinterpret.
  • The AsciiDoc template escapes | as \| (via Sprig's replace), since |
    is the default AsciiDoc table-cell separator and is also common in regex
    alternation.

• Wire go test ./... into the test CI workflow so the new regression test
  actually gates (the workflow previously only regenerated the example).

• Update the example CRD and regenerate example/output.md.

No parser or model changes are required because the field model already
retains MinLength, MaxLength, and Pattern.

Testing

Added pkg/builder/templates_test.go and
pkg/builder/testdata/constraints.yaml.

The fixture puts each constraint on its own field so a guard branch cannot be
covered by an unrelated sibling, and its pattern carries a named capture group
(<, >), an entity-like sequence (&), and an alternation (|). The tests
verify, for each built-in template, that:

• Min Length, Max Length, and Pattern all render;
• the HTML templates HTML-escape <, >, and & (a negative assertion ensures
  the raw named-capture group does not leak into the HTML);
• the AsciiDoc template escapes | as \| (a negative assertion ensures no
  unescaped separator is emitted).

Verified with:

go test ./...

The three templates were also rendered through crdoc to confirm the
generated Markdown, front-matter Markdown, and AsciiDoc output.

Fixes #329

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