Update authentication for PRONOTE >= 2026.2.5.6

[iGameCreep]

Since PRONOTE 2026.2.5.6, the authentication challenge logic has changed.
See diff between ObjetMoteurConnexion-2025.2.5.2 & ObjetMoteurConnexion-2025.2.5.6.

Previous logic:

• Decrypt given challenge with temporary key
• "Solve it" (remove characters on every pair index)
• Re-encrypt the solved challenge with temporary key

New logic:

• Encrypt given challenge with temporary key

Changed required:

• Edit src/structures/Challenge.ts to implement new challenge logic.

[linear]

NOTE-24