fix(spec-api): ignore XML declaration in validateXmlBodyEquals

[Copilot]

Since the migration from xml2js to fast-xml-parser (#10239), validateXmlBodyEquals prepends <?xml version='1.0' encoding='UTF-8'?> to plain expected strings, and fast-xml-parser (unlike xml2js) preserves that declaration as a ?xml node in the parsed output. This causes comparison failures when the actual request body omits the declaration.

Changes

• packages/spec-api/src/request-validations.ts — Add ignoreDeclaration: true to XMLParser options so declarations are stripped from both sides before comparison, restoring the semantic-equality behavior of xml2js.
• packages/spector/test/xml-validation.test.ts — Add test for the inverse path: actual body without a declaration matching an expected string (the exact scenario failing in Spector).

// Previously threw ValidationError; now passes
validateXmlBodyEquals(
  { rawBody: "<Root><a>1</a></Root>" },
  "<Root><a>1</a></Root>",
);

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commit: e9991db