Feature: Billing and usage statistics

[hundredwz]

What problem does this PR solve?

Issue Number: resolve #45

Problem Summary:
ML Agent lacks token usage tracking and billing functionality. Unlike Math Agent which uses ReActAgent throughout, ML Agent uses EvoCoder for code generation which bypasses the framework's built-in token tracking.

What is changed and the side effects?

Changed:

• Add token usage tracking to EvoCoder (accumulate tokens from LLM calls)
• Propagate token statistics through MLExecutorAgent, MLPlannerAgent, and MLSummaryAgent
• Add token_usage parameter to eda_tool to capture EDA-related token consumption

Side effects:

• Performance effects: None
• Breaking backward compatibility: None (all new fields have default values)

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
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