Add arithmetic calculator example using tool use

[omid-ant]

Adds examples/arithmetic_calc.py — a self-contained example demonstrating how to build a simple arithmetic calculator using Claude's tool use feature.

The example defines four tools (add, subtract, multiply, divide) and runs an agentic loop that lets Claude chain multiple operations to evaluate an expression. Key design decisions:

• Shared _AB_SCHEMA constant eliminates schema duplication across the three tools with identical signatures
• Loop exits on any non-tool_use stop reason (handles end_turn, max_tokens, refusal, etc.) rather than only exiting on end_turn, avoiding an infinite loop on unexpected stop reasons
• _run_tool handles divide-by-zero and unknown tool names explicitly
• Follows existing example conventions (from anthropic import Anthropic, ToolParam/MessageParam type annotations)