Implement the NATO converter with tests and Doxygen comments

Task: MIL-002#2
Refs #9
Refs #10
Refs #11
Refs #12
Refs #13

tests/test_main_module.py

@@ -0,0 +1,30 @@
+import subprocess
+import sys
+
+from nato_alphabet import constants
+
+
+def _run_module(stdin_text: str) -> subprocess.CompletedProcess[str]:
+    return subprocess.run(
+        [sys.executable, "-m", "nato_alphabet"],
+        input=stdin_text,
+        capture_output=True,
+        text=True,
+        check=False,
+        timeout=30,
+    )
+
+
+def test_python_dash_m_spells_word_from_piped_input() -> None:
+    result = _run_module("Hi\n")
+
+    assert result.returncode == 0
+    assert constants.RESULT_TEXT in result.stdout
+    assert result.stdout.splitlines()[-2:] == ["Hotel", "India"]
+
+
+def test_python_dash_m_exits_cleanly_on_end_of_input() -> None:
+    result = _run_module("")
+
+    assert result.returncode == 0
+    assert result.stderr == ""