fix: tests on MacOS

[SeverinAlexB]

Summary

• Bind all test DHT nodes and clients to 127.0.0.1 instead of 0.0.0.0 to fix 46 test failures on macOS
• Replace deprecated Testnet::new_async(N) / Testnet::new(N) with Testnet::builder(N).build() (defaults to localhost)
• Add .bind_address(Ipv4Addr::LOCALHOST) to all test DhtBuilder configs

Context

On newer macOS, binding UDP sockets to 0.0.0.0 causes DHT nodes to become unreachable — the OS may route UDP responses through a different interface than loopback, resulting in NoClosestNodes and UnexpectedResponses errors. This caused 46 out of ~96 tests to fail.

Mainline 6.1.1 added Testnet::builder(N) which defaults bind_address to 127.0.0.1 (vs 0.0.0.0 in the legacy new() / new_async() constructors). This PR adopts that pattern across all pkarr tests, matching what mainline's own test suite does.

Changes

• pkarr/src/client/tests.rs: Replaced all Testnet::new_async() calls with Testnet::builder().build(), added .dht(|b| b.bind_address(Ipv4Addr::LOCALHOST)) to the shared builder() helper and standalone client builders
• pkarr/src/extra/endpoints/mod.rs: Same pattern for all 5 endpoint tests
• relay/src/lib.rs: Added bind_address(LOCALHOST) to run_test() and run_testnet(), replaced Testnet::new() with Testnet::builder().build()

See pubky/mainline#82

[afterburn]

@SeverinAlexB lgtm, one small nit but feel free to ignore