Add idiomatic documentation to Go, Java, Python, and Rust clients

clients/python/src/mxgateway/auth.py

@@ -14,13 +14,16 @@ class ApiKey:
     value: str
 
     def __post_init__(self) -> None:
+        """Validate that the API key value is non-empty."""
         if not self.value:
             raise ValueError("api_key must not be empty")
 
     def __repr__(self) -> str:
+        """Return a repr that redacts the secret value."""
         return f"{type(self).__name__}({REDACTED!r})"
 
     def bearer_value(self) -> str:
+        """Return the value formatted as an HTTP `Bearer` token."""
         return f"Bearer {self.value}"