Add tests for echo example and address validation utilities

- Introduced `test_echo_thin_waist.py` to validate the echo example's output for Thin Waist lines.
- Added `test_address_validation.py` to cover functions for available interfaces, optimal binding addresses, and wildcard address expansion.
- Included parameterized tests and environment checks for IPv6 support.

tests/examples/test_echo_thin_waist.py

@@ -0,0 +1,51 @@
+import asyncio
+import contextlib
+import subprocess
+import sys
+import time
+from pathlib import Path
+
+import pytest
+
+# This test is intentionally lightweight and can be marked as 'integration'.
+# It ensures the echo example runs and prints the new Thin Waist lines.
+
+EXAMPLES_DIR = Path(__file__).parent.parent.parent / "examples" / "echo"
+
+
+@pytest.mark.timeout(20)
+def test_echo_example_starts_and_prints_thin_waist(monkeypatch, tmp_path):
+    # We run: python examples/echo/echo.py -p 0
+    cmd = [sys.executable, str(EXAMPLES_DIR / "echo.py"), "-p", "0"]
+    proc = subprocess.Popen(
+        cmd,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+        text=True,
+    )
+    assert proc.stdout is not None
+
+    found_selected = False
+    found_interfaces = False
+    start = time.time()
+
+    try:
+        while time.time() - start < 10:
+            line = proc.stdout.readline()
+            if not line:
+                time.sleep(0.1)
+                continue
+            if "Selected binding address:" in line:
+                found_selected = True
+            if "Available candidate interfaces:" in line:
+                found_interfaces = True
+            if "Waiting for incoming connections..." in line:
+                break
+    finally:
+        with contextlib.suppress(ProcessLookupError):
+            proc.terminate()
+        with contextlib.suppress(ProcessLookupError):
+            proc.kill()
+
+    assert found_selected, "Did not capture Thin Waist binding log line"
+    assert found_interfaces, "Did not capture Thin Waist interfaces log line"