Fix: retry on invalid JSON from Gemini

[runarmod]

Sometimes, the response from Gemini (and I assume the other LLM services) is not in a valid JSON format. This makes the json.loads-call throw an exception, and the service-call returns an empty dict.

This is frustrating, as the resulting markdown will have missing image descriptions, tables that have not been refined, etc. Since the error is ignored and the program moves on, one has to carefully check the logs during conversion, and then rerun the conversion if it happens.

IMHO, the most logical thing to do if the LLM service returns malformed JSON is to try again, in the same way we do if we are rate limited.

Note: this fix only handles the problem for the Gemini service. LMK if you want me to fix this for the other services as well.

Logs where this happens:

[...]
2025-08-07 11:10:27,331 - httpx - INFO - HTTP Request: POST https://generativelanguage.googleapis.com/v1beta/models/gemini-2.0-flash:generateContent "HTTP/1.1 200 OK"
2025-08-07 11:10:27,335 - google_genai.models - INFO - AFC remote call 1 is done.
2025-08-07 11:10:27,336 - marker - ERROR - Exception: Unterminated string starting at: line 3 column 21 (char 160)
LLMTableProcessor running: 4it [00:24,  4.96s/it]Traceback (most recent call last):
  File "C:\Users\[...]\.venv\Lib\site-packages\marker\services\gemini.py", line 79, in __call__
    return json.loads(output)
           ^^^^^^^^^^^^^^^^^^
  File "C:\Users\[...]\AppData\Roaming\uv\python\cpython-3.12.11-windows-x86_64-none\Lib\json\__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\[...]\AppData\Roaming\uv\python\cpython-3.12.11-windows-x86_64-none\Lib\json\decoder.py", line 338, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\[...]\AppData\Roaming\uv\python\cpython-3.12.11-windows-x86_64-none\Lib\json\decoder.py", line 354, in raw_decode
    obj, end = self.scan_once(s, idx)
               ^^^^^^^^^^^^^^^^^^^^^^
json.decoder.JSONDecodeError: Unterminated string starting at: line 3 column 21 (char 160)
2025-08-07 11:10:27,928 - google_genai.models - INFO - AFC is enabled with max remote calls: 10.
2025-08-07 11:10:29,877 - httpx - INFO - HTTP Request: POST https://generativelanguage.googleapis.com/v1beta/models/gemini-2.0-flash:generateContent "HTTP/1.1 200 OK"
[...]

[VikParuchuri]

Thanks for the fix! Would definitely take a PR for the other services if you have time