fix: use random port for Anvil instead of hardcoded 61611

[grumbach]

Summary

• Remove hardcoded ANVIL_DEFAULT_PORT = 61611, use OS-assigned random port by default
• Replace all .expect() in testnet.rs with proper Result propagation via TestnetError
• Replace all direct keys()[N] indexing with .first()/.get(N) safe access
• Make start_node(), Testnet::new(), default_wallet_private_key(), and all deploy_* functions return Result
• Fix test_run_multiple_testnets_in_parallel to use tokio::join! for actual concurrent testing
• Use inline format variables and top-level imports

Problem

1. Hardcoded port 61611 causes Address already in use (os error 48) failures in parallel/sequential tests
2. .expect() calls in non-test production code violate project rules and panic on setup failures
3. Direct keys()[N] indexing panics on index-out-of-bounds

Breaking changes

• start_node() now returns Result<(AnvilInstance, Url), TestnetError>
• Testnet::new() now returns Result<Self, TestnetError>
• default_wallet_private_key() now returns Result<String, TestnetError>
• deploy_network_token_contract(), deploy_data_payments_contract(), deploy_merkle_payments_contract() now return Result

Downstream callers (ant-node, ant-client) need to add .unwrap() in test code or propagate errors in production code.

Test results (with this fix)

All tests run with --test-threads=4 (parallel):

Suite	Tests	Result
ant-node: attack tests (security + merkle)	10	all pass
ant-node: chunk tests	11	all pass
ant-client: e2e_chunk	9	all pass
ant-client: e2e_payment	8	all pass
ant-client: e2e_merkle (35-node testnet)	2	all pass