feat(kernel): implement token budget tracking helpers (#1475)

[SH20RAJ]

This PR implements missing token-based helper methods for BudgetStatus as identified in the GSoC 2026 ideas list (General Framework Improvements).

Changes

• Implemented remaining_session_tokens(), remaining_daily_tokens(), remaining_monthly_tokens(), and remaining_total_tokens().
• Added comprehensive unit tests for token budget tracking logic.
• Standardized API for token-based tracking to improve developer experience.

Verification

• cargo test -p mofa-kernel --lib budget::tests passed.

Motivation

Part of the effort to improve framework-level cost attribution and budget management for AI agents.

[SH20RAJ]

Hi! I've implemented the missing token-based helper methods for BudgetStatus. This allows agent designers to easily track token consumption across session, daily, and monthly boundaries, providing finer-grained cost control as suggested in Idea 31. (Fixes #1475)

[SH20RAJ]

Supporting 'General Framework Improvements', this PR implements missing token budget helper methods (remaining_session_tokens, etc.) in BudgetStatus. These additions are crucial for fine-grained cost attribution and usage tracking for AI agents, as outlined in the GSoC ideas list. 💰