Null notes/chords in a Chord or Phrase are accepted, then crash later with NullReferenceException in ToString() and SaveAsync #187

What's wrong

Chord and Phrase check their collections for null but never check the elements inside them:

• Chord(IEnumerable<Note>) is in Keybinding/Models/MusicalTypes.cs around lines 150-159.
• Phrase(IEnumerable<Chord>) is around lines 359-368.
• Phrase(Chord) (around line 374) calls this([chord]) with no Ensure.NotNull, unlike Chord(Note).
• The [JsonConstructor] overloads of Chord, Phrase and Profile (Models/Profile.cs:41-50) all go through these constructors.

A bad value is therefore accepted when it is built and only fails later, somewhere unrelated.

Reproduction

Each case below was run against the current main:

• JsonSerializer.Deserialize<Chord>("{\"Notes\":[null]}") succeeds, giving Notes.Count == 1 and Notes[0] == null. Calling ToString() on the result then throws NullReferenceException in IsModifierNote.
• {"Notes":[null,{"Key":"A"}]} throws NullReferenceException from the OrderBy key selector instead of a JsonException.
• {"Notes":null} throws ArgumentNullException and {"Notes":[]} throws ArgumentException. Callers that catch JsonException miss both.
• new Phrase((Chord)null!) and Deserialize<Phrase>("{\"Sequence\":[null]}") both succeed, and ToString() then throws NullReferenceException.
• A profile loaded from {"Id":"a","Name":"A","Chords":{"x.y":{"Notes":[null]}}} reports ChordCount == 1. Once it is registered, KeybindingManager.SaveAsync → JsonKeybindingRepository.SaveProfileAsync throws NullReferenceException at n.ToString() (Services/JsonKeybindingRepository.cs:66), and the save fails part-way through.

NoteJsonConverter already wraps invalid keys in JsonException (Models/NoteJsonConverter.cs:318-328). That is the contract the other types should follow.

Suggested fix

• In the Chord and Phrase constructors, reject null elements with ArgumentException before Distinct/OrderBy.
• Add Ensure.NotNull(chord) to Phrase(Chord).
• In the private [JsonConstructor] overloads, catch ArgumentException and rethrow it as JsonException (inner exception kept), the same way NoteJsonConverter does.

Acceptance criteria

• Deserializing Chord or Phrase JSON with null, missing or empty Notes/Sequence throws JsonException.
• Constructing either type with a null element throws ArgumentException.
• Regression tests cover each case above.