fix: UTF-8 config encoding, pairing hint, credential_pool key, header normalization (#7174)

Four small fixes: (1) UTF-8 encoding for config open (@zhangchn #7063), (2) pairing hint placeholders (@konsisumer #7057), (3) missing credential_pool in cheap route (@kuishou68 #7025), (4) case-insensitive rate limit headers (@kuishou68 #7019).

cli.py

@@ -319,7 +319,7 @@ def load_cli_config() -> Dict[str, Any]:
     # Load from file if exists
     if config_path.exists():
         try:
-            with open(config_path, "r") as f:
+            with open(config_path, "r", encoding="utf-8") as f:
                 file_config = yaml.safe_load(f) or {}
             
             _file_has_terminal_config = "terminal" in file_config