Fix: implement LaTeX character escaping for game labels and player names

[tanav8570]

Description:
This PR resolves a RuntimeError during PDF generation caused by unescaped LaTeX reserved characters in game labels, player names, and titles. For examples special characters like #,_ , $, %, { , }, etc.

Key Changes:

1. Implemented latex_special_char() to handle encoding of _, $, #, , and other reserved characters.
2. Updated gambit_layout_to_ef to pass all user-defined strings through this encoding layer.
3. Handled backslash replacement separately to prevent double-escaping issues.

Testing:
Verified with a test script using labels like ACTION_!1 and Testing$100. The PDF now compiles successfully where it previously failed.

[tanav8570]

Hi! I'm Tanav, a student at IIT Delhi. I'm very interested in the GSoC project for draw_tree (Idea-3) and wanted to start by fixing this LaTeX encoding issue I discovered while testing the library. Looking forward to your feedback!

[edwardchalstrey1]

Hi! I'm Tanav, a student at IIT Delhi. I'm very interested in the GSoC project for draw_tree (Idea-3) and wanted to start by fixing this LaTeX encoding issue I discovered while testing the library. Looking forward to your feedback!

Hi Tanav, thanks for your enthusiasm. Please before going further have a read of the contribution guidelines for GSoC: https://www.gambit-project.org/gsoc_2026/#contributor-guidance

Edit: If this is Tanav Gupta, we have seen your email reaching out and will respond as soon as we can, though please be advised we have a high volume of applications to get through :)