If you are using Codeigniter rewrite to remove index.php from URLs, try adding exceptions like this to htaccess:
RewriteCond $1 !^(index\.php|robots.txt|sitemap.xml|google644d08docc30vtsd.html) RewriteRule ^(.*)$ /index.php/$1 [L]
Using the htaccess you included, try using the conditional:
RewriteCond %{REQUEST_FILENAME} !^(google644d08docc30vtsd.html)$
Google also allows you to verify by adding a DNS record.
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