Clarify JsonIgnore behavior for IsTargeted property in CoreActivity

[Copilot]

Addresses a review question about whether [JsonIgnore] on IsTargeted suppresses deserialization in addition to serialization.

Behavior

[JsonIgnore] with no condition defaults to JsonIgnoreCondition.Always — the property is excluded from both serialization and deserialization. This is intentional:

• IsTargeted is not part of the BF Framework activity JSON schema; Teams never sends it in incoming webhook payloads
• ConversationClient reads activity.IsTargeted before activity.ToJson() — the ?isTargetedActivity=true query parameter is appended to the URL, not the JSON body
• For BF SDK compat (FromCompatActivity()), IsTargeted must be set manually after conversion — Microsoft.Bot.Schema.Activity has no equivalent concept

// IsTargeted controls the URL query param, not the JSON body
if (activity.IsTargeted)
    url += "?isTargetedActivity=true";

string body = activity.ToJson(); // isTargeted never appears here

The existing test IsTargeted_IsNotDeserializedFromJson explicitly validates this behavior.

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[rido-min]

@singhk97 yes, the JsonIgnore is intentional, for the next phase of Targeted Messages we will move the IsTargeted field to the Recipient, and I was planning to make it serializable at that point, so it can be used in the compat layer