fix: prevent null max_tokens when using gpt-5 / gpt-5-mini

[shota-kizawa]

Fix

• Added proper handling to ensure max_tokens is always set to a valid value instead of null.
• Verified that the request now succeeds with both gpt-5 and gpt-5-mini.

Related issue

Closes #103

[liukidar]

Hello,
this as it is would remove the possibility to pass max_tokens to any model.