Serialize types implementing only IReadOnlyDictionary<,> as mappings

[youdie006]

Motivation

Fixes #606. A type that implements only IReadOnlyDictionary<TKey, TValue> (and neither the non-generic IDictionary nor IDictionary<TKey, TValue>) is serialized as a sequence of { Key, Value } mappings instead of a single YAML mapping:

# actual (before)
- Key: key1
  Value: value1
- Key: key2
  Value: value2

# expected
key1: value1
key2: value2

FullObjectGraphTraversalStrategy.TraverseObject dispatches dictionaries via ObjectFactory.GetDictionary, which only recognises IDictionary<,>. A read-only-only dictionary therefore returns false there and falls through to the IEnumerable branch (IReadOnlyDictionary<,> is IEnumerable<KeyValuePair<,>>), so it is written as a list. The BCL ReadOnlyDictionary<,> happens to also implement IDictionary, so only custom read-only dictionary types are affected. The asymmetry is real: the deserialize side already handles non-IDictionary dictionary types (see the existing GenericDictionaryThatDoesNotImplementIDictionaryCanBeDeserialized test).

Solution

• ObjectFactoryBase.GetDictionary now also detects IReadOnlyDictionary<,> (after the existing IDictionary<,> check, so types implementing both keep the current path) and wraps the value in a new GenericReadOnlyDictionaryToNonGenericAdapter<TKey, TValue>.
• The new adapter mirrors the existing GenericDictionaryToNonGenericAdapter but takes an IReadOnlyDictionary<,>. Object-graph traversal (TraverseDictionary) only enumerates the adapter, so only GetEnumerator is implemented; the rest throw NotSupportedException, exactly like the existing adapter's unused members.

The regular IDictionary<,> path is unchanged.

Test

Added SerializeGenericReadOnlyDictionary, plus a GenericTestReadOnlyDictionary<,> test helper (implements only IReadOnlyDictionary<,>, mirroring the existing GenericTestDictionary). It asserts the read-only dictionary serializes to the same YAML as an equivalent Dictionary<,>.

• Before the fix: fails (serializes as a sequence).
• After the fix: passes.

Verified locally in Docker: dotnet test — the full YamlDotNet.Test suite passes (the only 2 failures, ConformsWithYamlSpec, are pre-existing and caused by the YAML spec test-suite submodule not being present in the shallow checkout; they fail identically without this change). The library also builds cleanly for netstandard2.0 and net8.0 (0 warnings).

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Disclosure: this fix was prepared with AI assistance (Claude). I reviewed it, confirmed the root cause in the traversal dispatch, and verified the red-green test and full suite myself.