BlockRun enables pay-per-request access to 30+ AI models via x402 micropayments.
Users pay with USDC on Base - no API keys needed, just a funded wallet.

Features:
- Full x402 v2 protocol implementation with EIP-712 signing
- Support for all major model providers via BlockRun:
  - Anthropic (Claude Sonnet 4, Opus 4, Haiku 3.5)
  - OpenAI (GPT-4o, GPT-4o-mini, o1, o3-mini)
  - Google (Gemini 2.0 Flash, Gemini 2.5 Pro)
  - DeepSeek (Chat, Reasoner)
  - xAI (Grok 2, Grok 3)
- Streaming support
- Tool calling support
- Complete test coverage

Usage:
```rust
use rig::providers::blockrun;

let client = blockrun::Client::from_env(); // reads BLOCKRUN_PRIVATE_KEY
let agent = client.agent(blockrun::CLAUDE_SONNET_4).build();
let response = agent.prompt("Hello!").await?;
```

Rig's HTTP client abstraction treats 402 responses as errors before the
x402 payment flow can intercept them. Fixed by using reqwest directly
for the initial request to properly handle the 402 response and extract
payment headers.

Changes:
- Use reqwest::Client for initial request to handle 402 status
- Parse payment requirements from x-payment-required header
- Sign EIP-712 authorization and retry with payment header
- Removed unused bytes::Bytes import

Tested end-to-end with real x402 payments:
- Claude Sonnet 4: ✓
- GPT-4o: ✓
- DeepSeek: ✓
- Tool calling: ✓

feat(providers): add BlockRun x402 micropayment provider

- Add comprehensive documentation for BlockRun x402 micropayment provider
- Include on-chain transaction proof with Base mainnet transactions
- Add latency benchmarks for Claude, GPT-4o, DeepSeek
- Document all available models (30+ across providers)
- Add quick start guide and tool usage examples
- Reference upstream PR #1294

- Feature BlockRun pay-per-request at top of README
- Add on-chain transaction proof table (Base mainnet)
- Include latency benchmarks (~3-5s per model)
- Document 30+ available models across providers
- Link to upstream PR #1294

- Add BlockRun pay-per-request section at top of README
- Include on-chain transaction proof table (Base mainnet)
- Add latency benchmarks (~3-5s per model)
- Document 30+ available models across all providers
- Link to upstream PR #1294