fix: prevent index out of bounds in get_code_block_range (#271)

Add bounds checking to prevent nil indexing when row parameter is
outsidethe valid range (less than 1 or greater than number of lines).

Error: attempt to index local 'line' (a nil value)

lua/blink/cmp/windows/lib/docs.lua

@@ -154,6 +154,7 @@ end
 --- @param row number
 --- @return number?, number?
 function docs.get_code_block_range(lines, row)
+  if row < 1 or row > #lines then return end
   -- get the start of the code block
   local code_block_start = nil
   for i = 1, row do