Revert "PYTHON-4575 Remove hostname length check"

This reverts commit a3e9cfc.

Author: aclark4life

pymongo/srv_resolver.py

@@ -84,6 +84,8 @@ def __init__(
         except Exception:
             raise ConfigurationError(_INVALID_HOST_MSG % (fqdn,)) from None
         self.__slen = len(self.__plist)
+        if self.__slen < 2:
+            raise ConfigurationError(_INVALID_HOST_MSG % (fqdn,))
 
     def get_options(self) -> Optional[str]:
         from dns import resolver

test/test_dns.py

@@ -183,6 +183,18 @@ def create_tests(cls):
 
 class TestParsingErrors(PyMongoTestCase):
     def test_invalid_host(self):
+        self.assertRaisesRegex(
+            ConfigurationError,
+            "Invalid URI host: mongodb is not",
+            self.simple_client,
+            "mongodb+srv://mongodb",
+        )
+        self.assertRaisesRegex(
+            ConfigurationError,
+            "Invalid URI host: mongodb.com is not",
+            self.simple_client,
+            "mongodb+srv://mongodb.com",
+        )
         self.assertRaisesRegex(
             ConfigurationError,
             "Invalid URI host: an IP address is not",