feat(python): handle McpError(-32042) for MCP payment challenges

[Bortlesboat]

Description

Ports TypeScript PR #1728 to the Python SDK. When an MCP server throws a -32042 error (SEP-1036 UrlElicitationRequired) with payment requirements in error.data, the Python client now catches and processes payment automatically — matching the TypeScript behavior.

Changes

• Add JSONRPC_PAYMENT_REQUIRED_CODE = -32042 constant to types.py
• Update extract_payment_required_from_error() and is_payment_required_error() in utils.py to handle both 402 and -32042 codes, including namespaced data.x402 payloads
• Wrap call_tool() and get_tool_payment_requirements() in both async and sync clients with try/except for McpError(-32042)
• Add 26 test cases covering extraction, detection, async client, sync client, and backward compatibility

Tests

python -m uv run pytest mcp/tests/test_mcperror.py -v
======================== 26 passed in 0.10s ========================

All existing MCP tests continue to pass (21 server tests, 19 server_sync tests).

[cb-heimdall]

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