fix(webchat): fetch full sidebar content for truncated history

Add a bounded `chat.message.get` gateway method so Control UI can fetch one display-normalized transcript message by id when an assistant history preview was truncated. Keep `chat.history` lightweight, reject oversized/hidden/missing rows with explicit unavailable reasons, and wire the WebChat side reader to request full content only for visible truncated assistant messages.

Also refresh the generated Swift gateway protocol models and document the new assistant-message side-reader behavior.

Closes #84651.
Related #53242.

Co-authored-by: NianJiuZst <3235467914@qq.com>

apps/shared/OpenClawKit/Sources/OpenClawProtocol/GatewayModels.swift

@@ -6880,6 +6880,54 @@ public struct ChatHistoryParams: Codable, Sendable {
     }
 }
 
+public struct ChatMessageGetParams: Codable, Sendable {
+    public let sessionkey: String
+    public let agentid: String?
+    public let messageid: String
+    public let maxchars: Int?
+
+    public init(
+        sessionkey: String,
+        agentid: String? = nil,
+        messageid: String,
+        maxchars: Int?)
+    {
+        self.sessionkey = sessionkey
+        self.agentid = agentid
+        self.messageid = messageid
+        self.maxchars = maxchars
+    }
+
+    private enum CodingKeys: String, CodingKey {
+        case sessionkey = "sessionKey"
+        case agentid = "agentId"
+        case messageid = "messageId"
+        case maxchars = "maxChars"
+    }
+}
+
+public struct ChatMessageGetResult: Codable, Sendable {
+    public let ok: Bool
+    public let message: AnyCodable?
+    public let unavailablereason: AnyCodable?
+
+    public init(
+        ok: Bool,
+        message: AnyCodable?,
+        unavailablereason: AnyCodable?)
+    {
+        self.ok = ok
+        self.message = message
+        self.unavailablereason = unavailablereason
+    }
+
+    private enum CodingKeys: String, CodingKey {
+        case ok
+        case message
+        case unavailablereason = "unavailableReason"
+    }
+}
+
 public struct ChatSendParams: Codable, Sendable {
     public let sessionkey: String
     public let agentid: String?