Fix escaped-quote detection in JSON cleanup for strings ending with a backslash

[rootkiller6788]

Problem

cleanup_json in langfun/core/structured/schema/json.py detects an escaped double quote by checking only the immediately preceding character:

elif c == '"' and json_str[i - 1] != '\\':

This is incorrect when a closing quote is preceded by an escaped backslash. In JSON, \\" (two backslashes followed by a quote) means the string value ends with a literal backslash and the quote is a real closing quote. The old check treated that quote as escaped, so the scanner never left the string state and the trailing } was skipped, producing:

ValueError: Malformated JSON: missing 1 closing curly braces.

This affects any JSON-protocol lf.parse / lf.query response whose string value ends with a backslash (for example a Windows path like "C:\\", a regex fragment, or a LaTeX string).

Fix

Replace the single-character lookback with a helper that counts consecutive backslashes. A quote is escaped only when it is preceded by an odd number of backslashes, which is the correct JSON rule:

def _is_unescaped_quote(json_str, i):
  num_backslashes = 0
  j = i - 1
  while j >= 0 and json_str[j] == '\\':
    num_backslashes += 1
    j -= 1
  return num_backslashes % 2 == 0

Test

Added test_parse_with_escaped_backslash verifying that {"result": "C:\\"} parses to C:\ (previously raised JsonError). Existing tests, including the escaped-quote case in test_parse_basics, continue to pass.