feat: add integration tests against real Tempo node

[brendanjryan]

Summary

Add integration tests that run against a local Tempo blockchain node, matching the coverage from mpp-rs.

Test Coverage

Tempo intent tests (test_tempo_integration.py — 8 tests):

• Credential creation with real transaction building
• Transaction credential verification (sign → submit → verify)
• Hash credential verification (pre-sent tx hash)
• Rejection of insufficient transfer amounts
• Rejection of wrong recipient addresses
• Premium charge ($1000 amount tier)
• Fee payer E2E charge roundtrip (client signs 0x76, server co-signs)
• Fee payer wrong recipient rejected pre-broadcast

Client↔server roundtrip tests (test_client_integration.py — 11 tests):

• Full payment roundtrip via PaymentTransport + verify_or_challenge()
• Receipt header parsing (status, method, reference validation)
• Free endpoint passthrough (no payment required)
• 402 response when no auth provided
• 402 when wrong auth scheme (Bearer) is sent
• 402 when malformed credential (garbage base64) is sent
• 402 when no matching payment method configured
• Multiple sequential payments with unique receipts
• Client balance decreases after payment (balanceOf before/after)
• Fee payer E2E charge with receipt validation
• Fee payer with no signer configured fails verification
• Fee payer balance accounting (client pays exact amount, no gas)

Infrastructure

• docker-compose.yml — local Tempo --dev node (multi-arch, 200ms blocks)
• Makefile targets: make node, make node-stop, make test-integration
• conftest.py — session-scoped account funding (faucet RPC with dev key fallback)
• Integration marker auto-skips when TEMPO_RPC_URL is unset

Running

make node              # start local Tempo node
make test-integration  # run integration tests
make node-stop         # stop node