Fix bug where full name clash (e.g. with generic collections) leads to incomplete metadata generation

[layomia]

Needs dedicated unit/e2e tests (not just relying on existing dynamic serializer tests).

Faulty LOC:

runtimelab/src/libraries/System.Text.Json/System.Text.Json.SourceGeneration/JsonSourceGenerator.cs

Line 56 in 4eb43df

(SerializableTypes ??= new Dictionary<string, Type>())[type.FullName] = type;

Made evident in:

runtimelab/src/libraries/System.Text.Json/tests/Serialization/Array.ReadTests.cs

Line 114 in 4eb43df

public async Task ReadByteListAsJsonArray()