Add azeth_get_balance_history tool for tracking balance over time

[0xtus]

Description

Add a new MCP tool azeth_get_balance_history that returns the balance history of a smart account over time, including deposits, withdrawals, and payment events.

Motivation

Agents managing budgets need to understand their spending patterns. Currently azeth_balance only returns the current balance. A history view helps agents make better economic decisions.

Suggested approach

1. Add a new tool definition in src/tools/account.ts
2. Use the existing azeth_history transaction log and aggregate balance changes
3. Return an array of { timestamp, balance, event } entries

Acceptance criteria

• Tool returns balance snapshots with timestamps
• Supports optional from and to date filters
• Zod schema validates all inputs
• Returns structured JSON matching the existing response format