chore: block network access from unit tests (#2732)

# What does this PR do?
this blocks network access for all `tests/unit/` tests.
`tests/integration/` are untouched.

it also introduces an `allow_network` marker to explicitly allow network
access.

## Test Plan
`./scripts/unit-tests.sh`

tests/unit/conftest.py

@@ -4,6 +4,17 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import pytest_socket
+
 # We need to import the fixtures here so that pytest can find them
 # but ruff doesn't think they are used and removes the import. "noqa: F401" prevents them from being removed
 from .fixtures import cached_disk_dist_registry, disk_dist_registry, sqlite_kvstore  # noqa: F401
+
+
+def pytest_runtest_setup(item):
+    """Setup for each test - check if network access should be allowed."""
+    if "allow_network" in item.keywords:
+        pytest_socket.enable_socket()
+    else:
+        # Allowing Unix sockets is necessary for some tests that use local servers and mocks
+        pytest_socket.disable_socket(allow_unix_socket=True)