anchor_value: Fix parsing of large anchor values.

This handles the conditions when anchor values are larger than
LARGER_THAN_MAX_MESSAGE_ID by clamping them down to it.  Also added
tests for the function parse_anchor_value.

Fixes #16768.

zerver/views/message_fetch.py

@@ -839,6 +839,8 @@ def parse_anchor_value(anchor_val: Optional[str],
         anchor = int(anchor_val)
         if anchor < 0:
             return 0
+        elif anchor > LARGER_THAN_MAX_MESSAGE_ID:
+            return LARGER_THAN_MAX_MESSAGE_ID
         return anchor
     except ValueError:
         raise JsonableError(_("Invalid anchor"))