fix: 保留 PaC YAML 标量空白

仅规范结构性 mapping 容器头，并以完整 YAML round-trip 校验失败闭合。\n\nCo-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/src/platform-infra-pipelines-as-code-source-artifact.ts

@@ -1032,12 +1032,33 @@ function parseYamlRecord(text: string, label: string): Record<string, unknown> {
 }
 
 export function pacSourceArtifactYaml(value: Record<string, unknown>): string {
-  const serialized = Bun.YAML.stringify(value, null, 2)
-    .split("\n")
-    .map((line) => line.replace(/[\t ]+$/u, ""))
-    .join("\n")
-    .trim();
-  return `${serialized}\n`;
+  const lines = Bun.YAML.stringify(value, null, 2).split("\n");
+  let blockScalarHeaderIndent: number | null = null;
+  const normalized = lines.map((line) => {
+    const content = line.trim();
+    const indent = line.length - line.trimStart().length;
+    if (blockScalarHeaderIndent !== null) {
+      if (content.length === 0 || indent > blockScalarHeaderIndent) return line;
+      blockScalarHeaderIndent = null;
+    }
+
+    const structuralLine = line.replace(/:[\t ]+$/u, ":");
+    if (/(?:^|:[\t ]+|-[\t ]+)[|>](?:[1-9][+-]?|[+-][1-9]?|[+-])?[\t ]*(?:#.*)?$/u.test(structuralLine)) {
+      blockScalarHeaderIndent = indent;
+    }
+    return structuralLine;
+  }).join("\n");
+  const withSingleEofNewline = `${normalized.replace(/\n+$/u, "")}\n`;
+  const roundTrip = parseYamlRecord(withSingleEofNewline, "serialized source artifact");
+  const roundTripDrift = firstCanonicalDrift(value, roundTrip, "$");
+  if (roundTripDrift !== null) {
+    throw new PacSourceArtifactError("source-artifact-yaml-round-trip-failed", {
+      driftPath: roundTripDrift.path,
+      expectedEvidence: roundTripDrift.expected,
+      actualEvidence: roundTripDrift.actual,
+    });
+  }
+  return withSingleEofNewline;
 }
 
 function requiredParam(binding: PacSourceArtifactBinding, key: string): string {