fix(utils): don't warn when network is unreachable in _get_ip

[fwh888]

Summary

LocalCluster(processes=False) on a machine without external connectivity emits a spurious RuntimeWarning every time it starts:

RuntimeWarning: Couldn't detect a suitable IP address for reaching "8.8.8.8", defaulting to hostname: [Errno 101] Network is unreachable

The UDP probe in _get_ip raises OSError when the default probe host 8.8.8.8 is unreachable (offline machine). This is an expected, recoverable condition — the hostname_fallback() path already yields a usable local address.

Changes

In distributed/utils.py, _get_ip now silently downgrades to the hostname fallback for network-unreachable errors (ENETUNREACH, EHOSTUNREACH, ENETDOWN) instead of emitting a RuntimeWarning. Genuine socket errors (e.g. EACCES) still warn as before.

Tests

Added unit tests in tests/test_utils.py:

• test_get_ip_silent_when_network_unreachable — asserts no warning is raised on ENETUNREACH
• test_get_ip_warns_for_other_socket_errors — asserts EACCES still warns
• test_get_ip_returns_local_address — normal probe path unchanged

All updated tests pass; ruff and pre-commit clean on touched files.

Fixes #8559

[fwh888]

Hi maintainers 👋 — a quick note on the current CI status to make review easier:

Green (relevant to this PR):

• pre-commit hooks ✅ (ruff format/lint + all hooks)
• Build (and upload) ✅
• docs/readthedocs.org:distributed ✅
• 35+ test-matrix jobs ✅ (arm/mac/mindeps + several py versions, both ci partitions)

The 3 failures are unrelated to this change:

• nightly test (×2) fails at pixi install (dependency resolution) — the same failure exists on main 16c1bdc/82137211.
• ubuntu-latest py310 test-noqueue not ci1 fails only on test_shuffle.py::test_restarting_during_transfer_raises_killed_worker — a shuffle-transfer timing/flakiness case, with no relation to get_ip or the error-path change here.

The change itself is narrow: _get_ip now silently falls back to the hostname when the UDP probe fails with ENETUNREACH/EHOSTUNREACH/ENETDOWN, with 3 dedicated unit tests (silent downgrade, preserved warning for other OSError, unchanged normal path).

Happy to rerun or adjust anything. Thanks!

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    40 files  ± 0      40 suites  ±0   14h 43m 21s ⏱️ + 24m 23s
 4 163 tests + 3   3 983 ✅ + 3    179 💤 ±0  1 ❌ ±0 
81 022 runs  +60  76 779 ✅ +60  4 242 💤 ±0  1 ❌ ±0 

For more details on these failures, see this check.

Results for commit 92c89ad. ± Comparison against base commit dc182bd.