fix: skip 429 rate limit errors from GitHub domains

[Copilot]

GitHub URLs returning 429 (Too Many Requests) after retry exhaustion were being reported as broken links, causing false positives in CI.

Changes

• After all retries exhausted, 429 responses from GitHub domains are now marked as skipped instead of broken
• Uses proper hostname parsing to match github.com, *.github.com, and *.githubusercontent.com
• Prevents false matches on domains like fakegithub.com or evil.com/github.com/path

// After retries exhausted
if (lastResult && lastResult.status === 429) {
  const hostname = new URL(url).hostname;
  if (hostname === 'github.com' || 
      hostname.endsWith('.github.com') || 
      hostname.endsWith('.githubusercontent.com')) {
    return { valid: true, skipped: true, reason: 'GitHub 429 - Rate limited, skipped' };
  }
}

Non-GitHub 429s and other GitHub error codes (404, 500, etc.) continue to be reported as failures.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>fix: ignore all 429s on github</issue_title>
<issue_description>after #2384 there are still 429s from github

[429 - Too Many Requests] https://github.com/celestiaorg/celestia-app/blob/2b8cc9e23826ccb658b7dd5aa6cd51a0921a0c29/go.mod#L35 (in app/operate/data-availability/ibc-relayer/page.mdx)
```</issue_description>

<agent_instructions>this should allow skipping all 429s on github urls</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes fix: ignore all 429s on github #2386

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