fix: . trigger prefix extraction broken when . not yet in document

When nvim-cmp sends completion before didChange commits the ., the
cursor col is at the word end, but the old code always scanned from
col-1, cutting off the variable's last character (instTestTime vs
instTestTimer). Now checks if the character at col is . or empty;
if so, scans from col-1; otherwise scans from col.

src/main.lua

@@ -848,11 +848,19 @@ function handlers.textDocument_completion(params)
 				end
 			end
 		elseif trigger == "." then
-			local prefix_start = col - 1
+			-- Extract word before cursor, handling both cases:
+			-- 1) . IS in doc: cursor is after ., scan from col-1
+			-- 2) . NOT in doc yet: cursor is at word end, scan from col
+			local scan_end = col
+			local c = col > 0 and line:sub(col, col) or ""
+			if c == "" or c == "." then
+				scan_end = col - 1
+			end
+			local prefix_start = scan_end
 			while prefix_start > 0 and line:sub(prefix_start, prefix_start):match("[%w_]") do
 				prefix_start = prefix_start - 1
 			end
-			local prefix = line:sub(prefix_start + 1, col - 1)
+			local prefix = scan_end > prefix_start and line:sub(prefix_start + 1, scan_end) or ""
 
 			if prefix ~= "" then
 				local var_name = prefix:gsub("^#", "")