_get_firecrawl_client only read api_key from the tool config and ignored
base_url, so self-hosted Firecrawl deployments were unreachable: the SDK
defaulted to https://api.firecrawl.dev and raised 'No API key provided'.
Now base_url is forwarded as api_url; api_url is omitted when unset so
cloud behavior is unchanged.
Also document the optional self-hosted base_url on both Firecrawl entries
in config.example.yaml and reconcile their headers to the fastCRW house
style (Cloud requires FIRECRAWL_API_KEY; self-host may need no key.).