docs: update test format documentation in README

Update documentation to reflect new TXT format with separator for summarization tests instead of JSON format. Clarify that expected field may be empty if summary generation fails.

feat: change test generation to TXT format with separator

Change test generation from JSON to TXT format with TEST_SEPARATOR. Add filename sanitization function to handle MongoDB record IDs. Update output path and file naming logic. Add attempt to generate expected summary through LLM with fallback to empty string.

tests/codegen/test1.txt

@@ -1,7 +1,6 @@
-Write a Python function that calculates the factorial of a number using recursion.
+Write a Python function that calculates the factorial of a number.
 ==============
 def factorial(n):
-    if n == 0 or n == 1:
+    if n == 0:
         return 1
-    else:
-        return n * factorial(n-1)
+    return n * factorial(n - 1)