fix(leetcode.py): lock the csrf token as the server accepts it

[kaifcodec]

• This PR uses a static CSRF token in the module, as the server currently accepts it and it does not appear to expire.
• It has been working reliably for more than 72 hours.

Observations:

• There is no observed GET request from the client that returns this token in the response.
• Across multiple different sessions, this token remains the same and appears to be static.