fix(slack): restore or-chain for user_name/full_name to match upstream

Round-2 fixup over-applied Port Rule #1. Upstream JS uses || chain
intentionally — empty-string username falls through to name/user_id.
The is-not-None chain preserved "" and diverged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: patrick-chinchill

src/chat_sdk/adapters/slack/adapter.py

@@ -1030,8 +1030,11 @@ async def _handle_block_suggestion(
         user_id = user_ref.get("id", "")
         username = user_ref.get("username")
         name = user_ref.get("name")
-        user_name = username if username is not None else (name if name is not None else user_id)
-        full_name = name if name is not None else (username if username is not None else user_id)
+        # Upstream uses `||` truthy-fallthrough intentionally: an empty-string
+        # username falls through to name, then user_id. See upstream
+        # packages/adapter-slack/src/index.ts lines ~1258-1260.
+        user_name = username or name or user_id
+        full_name = name or username or user_id
 
         action_id = payload.get("action_id", "")
         val = payload.get("value")