Implement the NATO converter with tests and Doxygen comments

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

tests/application/test_spell_word_session.py

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+from collections.abc import Mapping
+
+from nato_alphabet.application.ports import InputClosedError
+from nato_alphabet.application.spell_word_session import SpellWordSession
+
+
+class FakeSource:
+    def load(self) -> Mapping[str, str]:
+        return {"A": "Alfa", "B": "Bravo"}
+
+
+class FakeConsole:
+    def __init__(self, words: list[str]) -> None:
+        self._words = words
+        self.asked = 0
+        self.invalid = 0
+        self.shown: list[list[str]] = []
+
+    def ask_word(self) -> str:
+        self.asked += 1
+        if not self._words:
+            raise InputClosedError
+        return self._words.pop(0)
+
+    def show_invalid(self) -> None:
+        self.invalid += 1
+
+    def show_code_words(self, code_words: list[str]) -> None:
+        self.shown.append(code_words)
+
+
+def test_run_shows_code_words_for_valid_word() -> None:
+    console = FakeConsole(["ab"])
+
+    SpellWordSession(FakeSource(), console).run()
+
+    assert console.shown == [["Alfa", "Bravo"]]
+    assert console.asked == 1
+
+
+def test_run_asks_again_after_invalid_word() -> None:
+    console = FakeConsole(["a1", "", "b"])
+
+    SpellWordSession(FakeSource(), console).run()
+
+    assert console.invalid == 2
+    assert console.asked == 3
+    assert console.shown == [["Bravo"]]
+
+
+def test_run_ends_cleanly_when_input_is_closed() -> None:
+    console = FakeConsole([])
+
+    SpellWordSession(FakeSource(), console).run()
+
+    assert console.shown == []