feat: introduce headings audit in preflight

[radhikagpt1208]

This PR adds headings issues detection to the preflight audit. When a webpage is scraped, all heading elements are validated and are reported with SEO impact, actionable recommendations and AI generated suggestions.

The headings audit in preflight will now return opportunities in this format:

{
	"result": [
      {
        "pageUrl": "https://author-p125761-e1234605.adobeaemcloud.com/content/aso-preflight/index/preflight-headings-test.html",
        "step": "suggest",
        "audits": [
          {
            "name": "headings",
            "type": "seo",
            "opportunities": [
              {
                "check": "heading-missing-h1",
				 "seoRecommendation": "Pages should have exactly one H1 element for SEO and accessibility.",
				 "issue": "Missing H1 Heading",
				 "issueDetails": "Page does not have an H1 element",
                "seoImpact": "High",
				 "aiSuggestion": "Explore Pain Perception: Insights and Reflections"
				},
				{
					"check": "heading-order-invalid",
					"seoRecommendation": "Heading levels should increase by one (example: H1→H2), not jump levels (example: H1→H3).",
					"issue": "Invalid Heading Order",
					"issueDetails": "Heading hierarchy skips levels.",
					"seoImpact": "Moderate",
					"suggestion": "Adjust heading levels to maintain proper hierarchy."
				},
				{
					"check": "heading-order-invalid",
					"seoRecommendation": "Heading levels should increase by one (example: H1→H2), not jump levels (example: H1→H3).",
					"issue": "Invalid Heading Order",
					"issueDetails": "Heading hierarchy skips levels.",
					"seoImpact": "Moderate",
					"suggestion": "Adjust heading levels to maintain proper hierarchy."
				}
            ]
          }
        ]
      }
    ]
}

Please ensure your pull request adheres to the following guidelines:

• make sure to link the related issues in this description
• when merging / squashing, make sure the fixed issue references are visible in the commits, for easy compilation of release notes
• If data sources for any opportunity has been updated/added, please update the wiki for same opportunity.

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[github-actions]

This PR will trigger a minor release when merged.

[solaris007]

🎉 This PR is included in version 1.247.0 🎉

The release is available on GitHub release

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