feat: Add BlockRun AI integration for agents

[1bcMax]

Summary

Add BlockRun AI integration - access 30+ AI models with x402 micropayments.

Usage

from ampersend_sdk.integrations.blockrun import BlockRunAI

async with BlockRunAI(treasurer=treasurer) as ai:
    response = await ai.chat("gpt-4o", "What is 2+2?")
    print(response)  # "4"

Features

• 30+ AI models (GPT-4o, Claude, Gemini, DeepSeek, Llama)
• Pay-per-request with USDC on Base
• No API keys needed - wallet is your identity
• Automatic x402 payment handling via Treasurer

Files

• src/ampersend_sdk/integrations/blockrun.py - BlockRunAI client
• tests/test_blockrun_integration.py - 9 unit tests
• examples/blockrun_ai_example.py - Usage example

Test plan

• All 51 tests pass
• Lint passes

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