fix(indexmap): harden sitemap pinging and indexnow key handling

[fidalgo]

Summary

Tighten correctness, determinism, and operability across the sitemap pinging flow.

Changes

IndexNow:

• enforce 32-character lowercase hex keys for configured and discovered keys
• reject key files whose contents do not exactly match the filename
• stop accepting trailing newlines in key files
• generate new keys in strict hex format
• make existing key-file reuse deterministic by sorting candidates
• include keyLocation in submission payloads when derivable
• return structured ping results and print explicit rake task output

Google:

• require exact Search Console property matches instead of substring matching
• memoize accessible site authorization checks to avoid repeated list_sites calls
• return structured ping results and print explicit rake task output
• downgrade unauthorized access logging to debug to keep test output clean

Writer:

• stop falling back to Time.now for sitemap index lastmod values
• omit section lastmod when entries do not provide one, keeping output deterministic

How to test

Release notes

Does this change need an explicit changelog or release-note callout beyond the PR title?

Checklist

• PR title is a Conventional Commit and suitable for squash merge
• Tests added/updated (if applicable)
• Docs updated (if applicable)
• I kept changes focused and easy to review

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