Apply UnknownDerivedTypeHandling.Fail to tag-based polymorphism

[fdcastel]

Motivation

When using tag-based polymorphism with UnknownDerivedTypeHandling.Fail, unknown property discriminators correctly throw a YamlException. However, unknown YAML tags are silently ignored — the deserializer falls through to the base type without any error.

This inconsistency makes it impossible to detect configuration errors when using tag-based type discrimination (e.g., !http, !ping, !tcp tags). A typo in a tag silently produces a base-type instance instead of failing fast.

Example

[YamlPolymorphic(DiscriminatorStyle = YamlTypeDiscriminatorStyle.Tag)]
[YamlDerivedType(typeof(Dog), Tag = \"!dog\")]
[YamlDerivedType(typeof(Cat), Tag = \"!cat\")]
class Animal { public string Name { get; set; } }

!lizard    # typo — should fail but silently deserializes as Animal
Name: Gecko

Solution

Apply the same Fail/FallBackToBase logic to the tag lookup path, consistent with how property-based discriminators already work:

1. YamlObjectConverter.ReadPolymorphic() — After the tag lookup fails, check DefaultDerivedType first, then check UnknownDerivedTypeHandling == Fail and throw.
2. Source generator — Emit the same check after the tag-matching if chain in the generated ReadValue methods.
3. YamlThrowHelper.ThrowUnknownTypeTag() — New helper for tag-specific error messages (e.g., \"Unknown type tag '!lizard' for 'Animal'\").

Tests

10 new tests covering:

• Unknown tag with default (Fail) handling via attributes
• Unknown tag with FallBackToBase via attributes
• Unknown tag with Fail via options-level DerivedTypeMappings
• Unknown tag with FallBackToBase via options
• Attribute-level override takes precedence over options
• Dictionary values with unknown tags
• Tag-only entries (no discriminator) with unknown tags
• Known tags still work correctly