[Bug]: A string value starting with a leading unescaped quote (""content) silently drops the escaped quote #220

Version of the library

0.63.3

Describe the bug

When a JSON string value begins with a literal, unescaped double quote
(i.e. the raw text is "key": ""content..., meaning: opening quote, then
immediately another quote that is meant to be a literal character, then
the rest of the content), repair_json treats the second quote as if the
string were empty, and silently drops it instead of escaping it.

This looks related to #46 (unescaped quote followed later by a comma), but
that issue is about a quote appearing in the middle of a string and is
now fixed in this version. This report is about the quote being the very
first character of the content, which still fails.

How to reproduce

>>> from json_repair import repair_json
>>> repair_json('{"title": ""hello" world"}')
'{"title": "hello\\" world\\""}'

Note the leading \" before hello is missing entirely, and the trailing
quote is duplicated/misplaced as a result. Compare with a control case
where the quote is not at the very start of the string, which repairs
correctly:

>>> repair_json('{"title": "hello \\"world\\" test"}')
'{"title": "hello \\"world\\" test"}'   # correct, quote is not at the start

The bug reproduces regardless of what follows (with or without a trailing
comma, with or without other keys):

>>> repair_json('{"title": ""hello" world", "b": "y"}')
'{"title": "hello\\" world", "b": "y"}'

>>> repair_json('{"a": "x", "title": ""hello" world"}')
'{"a": "x", "title": "hello\\" world\\""}'

This is common in real-world LLM output whenever a generated sentence
begins with a quoted phrase, e.g. a news headline like:

"title": ""We love our Americans": German town..."

Expected behavior

The leading quote should be treated as a literal character and escaped,
the same way a quote in the middle of the string is already handled:

>>> repair_json('{"title": ""hello" world"}')
'{"title": "\\"hello\\" world"}'

>>> repair_json('{"title": ""hello" world", "b": "y"}')
'{"title": "\\"hello\\" world", "b": "y"}'