Implement the NATO converter with tests and Doxygen comments

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

tests/domain/test_spelling.py

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+import pytest
+
+from nato_alphabet.adapters.csv_alphabet import load_nato_dict
+from nato_alphabet.domain.spelling import InvalidWordError, generate_phonetic
+
+ALPHABET = load_nato_dict()
+LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+
+@pytest.mark.parametrize("letter", list(LETTERS))
+def test_generate_phonetic_maps_letter_to_its_code_word(letter: str) -> None:
+    assert generate_phonetic(letter, ALPHABET) == [ALPHABET[letter]]
+
+
+def test_generate_phonetic_uses_known_code_words() -> None:
+    assert generate_phonetic("ADZ", ALPHABET) == ["Alfa", "Delta", "Zulu"]
+
+
+def test_generate_phonetic_covers_all_26_letters() -> None:
+    assert len(generate_phonetic(LETTERS, ALPHABET)) == len(LETTERS)
+
+
+def test_generate_phonetic_is_case_insensitive_for_mixed_case() -> None:
+    assert generate_phonetic("Hello", ALPHABET) == [
+        "Hotel",
+        "Echo",
+        "Lima",
+        "Lima",
+        "Oscar",
+    ]
+
+
+@pytest.mark.parametrize("word", ["a1", "two words", "hi!", "", "ß", "æ"])
+def test_generate_phonetic_raises_for_non_letter_or_empty(word: str) -> None:
+    with pytest.raises(InvalidWordError):
+        generate_phonetic(word, ALPHABET)
+
+
+def test_generate_phonetic_keeps_cause_of_invalid_character() -> None:
+    with pytest.raises(InvalidWordError) as info:
+        generate_phonetic("a1", ALPHABET)
+
+    assert isinstance(info.value.__cause__, KeyError)