Add maxTokenBudget with automatic abort

[ibrahimkzmv]

Issue:

• There is currently no mechanism to enforce a budget or abort when spending exceeds a threshold.
• A single agent stuck in a tool loop can consume thousands of tokens per turn.
• Users running against paid APIs (e.g., Anthropic, OpenAI) have no safeguards against runaway costs.
• Implementing token budget enforcement would be high-value and low-effort.

Proposed implementation:

• Allow passing a max_token_budget configuration value.

[ibrahimkzmv]

Hi @Yijia-Xiao
Before I spend time on a PR, I wanted to check if you think this feature is useful and aligns with the project goals. Would you be open to this addition?