fix: preserve include directive in output on error in ProcessIncludes

All error paths in the ReplaceAllFunc callback returned nil, which
ReplaceAllFunc treats as an empty replacement, silently erasing the
guard (if err != nil) fired, every subsequent include in the file
was also erased. Return spec (the original directive bytes) instead
so failed includes are preserved and subsequent ones are not lost.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

includes/templates.go

@@ -113,7 +113,7 @@ func ProcessIncludes(
 		contents,
 		func(spec []byte) []byte {
 			if err != nil {
-				return nil
+				return spec
 			}
 
 			groups := reIncludeDirective.FindSubmatch(spec)
@@ -143,7 +143,7 @@ func ProcessIncludes(
 						"unable to unmarshal template data config",
 					)
 
-				return nil
+				return spec
 			}
 
 			log.Tracef(vardump(facts, data), "including template %q", path)
@@ -151,7 +151,7 @@ func ProcessIncludes(
 			templates, err = LoadTemplate(base, includePath, path, left, right, templates)
 			if err != nil {
 				err = facts.Format(err, "unable to load template")
-				return nil
+				return spec
 			}
 
 			var buffer bytes.Buffer
@@ -163,7 +163,7 @@ func ProcessIncludes(
 					"unable to execute template",
 				)
 
-				return nil
+				return spec
 			}
 
 			recurse = true