fix: address Cubic round-6 P2/P3 issues

- P2: treat unknown part types as non-content in message-builder messageHasContentFromSDK
- P3: reuse shared isRecord from record-type-guard.ts in opencode-http-api

src/hooks/anthropic-context-window-limit-recovery/message-builder.ts

@@ -42,8 +42,6 @@ function messageHasContentFromSDK(message: SDKMessage): boolean {
     }
 
     if (TOOL_TYPES.has(type)) return true
-
-    return true
   }
 
   return false

src/shared/opencode-http-api.ts

@@ -1,12 +1,9 @@
 import { getServerBasicAuthHeader } from "./opencode-server-auth"
 import { log } from "./logger"
+import { isRecord } from "./record-type-guard"
 
 type UnknownRecord = Record<string, unknown>
 
-function isRecord(value: unknown): value is UnknownRecord {
-  return typeof value === "object" && value !== null
-}
-
 function getInternalClient(client: unknown): UnknownRecord | null {
   if (!isRecord(client)) {
     return null