* fix(community): normalize max_results before handing it to Firecrawl and fastCRW
* test(community): pin the explicit-null max_results shape
The parametrized `omitted` case reused None as its "key absent" marker, so
`extra["max_results"] = configured` never ran for None and the key-present,
value-null YAML shape never reached _coerce_max_results. Use a distinct
sentinel for absence and add an explicit-null case per provider.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>