Refactor MCP client initialization and fix method resolution in _mcp.py

[daatsuka]

Summary

Implements MCP client support and a generic x402-fetch wrapper as requested in #7, adding both synchronous and asynchronous variants with full test coverage.

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Changes

New files

src/x402_openai/_http.py (+109 lines)

• create_x402_session(wallet, ...) — synchronous requests.Session subclass that intercepts HTTP 402 responses, signs the x402 payment challenge, and retries automatically
• create_async_x402_session(wallet, ...) — async httpx.AsyncClient equivalent

src/x402_openai/_mcp.py (+331 lines)

• X402MCPClient — synchronous MCP JSON-RPC 2.0 client with x402 payment handling; supports context-manager usage
• AsyncX402MCPClient — async version using httpx.AsyncClient; supports async with
• Both clients handle result: null / non-dict results with explicit ValueError (JSON-RPC 2.0 §5)

src/x402_openai/_cli.py (+60 lines)

• create-x402-wallet entry-point: generates an EVM keypair, saves to ~/.x402/wallet.json with 0o600 permissions

Modified files

src/x402_openai/__init__.py (+20 / -1)

• Re-exports X402MCPClient, AsyncX402MCPClient, create_x402_session, create_async_x402_session
• Updated module docstring with usage examples for both HTTP and MCP clients

pyproject.toml (+3)

• Registered create-x402-wallet = "x402_openai._cli:main" as an installed script

Tests

File	Tests added
tests/test_http.py	+164 lines — 8 test cases covering sync/async sessions, retry on 402, non-402 passthrough, signing failure
tests/test_mcp.py	+353 lines — 18 test cases covering JSON-RPC dispatch, 402 retry, error handling, None/non-dict result guard
tests/test_transport.py	+3 / -1 — minor fixture update

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Local verification

$ pip install -e .
$ pytest tests/ -v
...
61 passed in 1.37s

All 61 tests pass (5 test modules: test_http, test_mcp, test_transport, test_wallet_factory, test_wallets).

Closes #7