Fix: generate JsonPatch for @dynamicModel with ...Record<unknown> + backcompat support

[Copilot]

Dynamic models with ...Record<unknown> (BinaryData additional properties) were silently skipping JsonPatch generation because SupportsBinaryDataAdditionalProperties short-circuited BuildJsonPatchField(). The model would fall back to a plain AdditionalProperties property instead of the dynamic patch mechanism.

Changes

• BuildJsonPatchField(): Remove SupportsBinaryDataAdditionalProperties guard — all @dynamicModel types get JsonPatch regardless of additional property type
• BuildProperties(): When JsonPatchProperty is set and SupportsBinaryDataAdditionalProperties is true, filter out AdditionalProperties (JsonPatch subsumes it for unknown AP) — unless backcompat is needed
• IsAdditionalBinaryDataParameter() helper: Extracted to detect parameters backed by _additionalBinaryDataProperties either via field or property backing field
• BuildConstructors(): Extended body-statement filtering to strip _additionalBinaryDataProperties assignments from both the full constructor and the init constructor when JsonPatch is being used
• NeedsBackCompatAdditionalProperties: New lazy property that checks if the previous contract (LastContractView) had an AdditionalProperties property. When true, both JsonPatch and AdditionalProperties are generated together to maintain backward compatibility
• UpdateFullConstructorParameters(): Simplified — patch parameter addition is factored out of the inner backcompat condition and always appended after the AdditionalBinaryData parameter check. In backcompat mode the old additionalProperties parameter is kept; in all cases patch is added after it.

Result

A @dynamicModel with ...Record<unknown> now generates JsonPatch only (no AdditionalProperties):

[Experimental("SCME0001")]
private JsonPatch _patch;

public DynamicModel(string name) { Name = name; }

internal DynamicModel(string name, in JsonPatch patch)
{
    Name = name;
    _patch = patch;
}

[JsonIgnore, EditorBrowsable(EditorBrowsableState.Never), Experimental("SCME0001")]
public ref JsonPatch Patch => ref _patch;

When the model was previously shipped with AdditionalProperties (backcompat), both are generated:

[Experimental("SCME0001")]
private JsonPatch _patch;
private protected readonly IDictionary<string, BinaryData> _additionalBinaryDataProperties;

public DynamicModel(string name) { Name = name; _additionalBinaryDataProperties = new ChangeTrackingDictionary<...>(); }

internal DynamicModel(string name, IDictionary<string, BinaryData> additionalProperties, in JsonPatch patch)
{
    Name = name;
    _additionalBinaryDataProperties = additionalProperties;
    _patch = patch;
}

public ref JsonPatch Patch => ref _patch;
public IDictionary<string, BinaryData> AdditionalProperties => _additionalBinaryDataProperties;

The serialization code for the backcompat case correctly:

• Writes both AdditionalProperties (iterated as BinaryData) and Patch.WriteTo() in JsonModelWriteCore
• Deserializes by initializing both IDictionary<string, BinaryData> additionalProperties and JsonPatch patch, then constructing new DynamicModel(p1, additionalProperties, patch)

Typed additional properties (...Record<string>, union types) are unaffected.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Need to handle models with both dynamicModel and AdditionalProperties</issue_title>
<issue_description>We currently don't generate the JsonPatch property for a dynamic model that also has additional unknown properties. e.g.

@dynamicModel
model DynamicModel {
  name: string;
  optionalUnknown?: unknown;
  optionalInt?: int32;
  foo: AnotherDynamicModel;
  listFoo: AnotherDynamicModel[];
  listOfListFoo: AnotherDynamicModel[][];
  dictionaryFoo: Record<AnotherDynamicModel>;
  dictionaryOfDictionaryFoo: Record<Record<AnotherDynamicModel>>;
  dictionaryListFoo: Record<AnotherDynamicModel[]>;
  listOfDictionaryFoo: Record<AnotherDynamicModel>[];
  ...Record<unknown>;
}

We can skip generating AdditionalProperties for such cases, but we will likely need backcompat support in case a model was already shipped with AdditionalProperties.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Need to handle models with both dynamicModel and AdditionalProperties #9526

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