chore: fix docstrings and tests

Signed-off-by: oGranny <ogranny.github.io@gmail.com>

Author: oGranny

src/hiero_sdk_python/consensus/topic_update_transaction.py

@@ -51,6 +51,9 @@ def __init__(
             auto_renew_period (Duration): The auto-renew period for the topic.
             auto_renew_account (AccountId): The account ID for auto-renewal.
             expiration_time (Timestamp): The expiration time of the topic.
+            custom_fees (list[CustomFixedFee]): A list of custom fees to set for the topic.
+            fee_schedule_key (Key): The fee schedule key for the topic.
+            fee_exempt_keys (list[Key]): A list of fee exempt keys for the topic
         """
         super().__init__()
         self.topic_id: TopicId | None = topic_id
@@ -95,10 +98,10 @@ def set_memo(self, memo: str) -> TopicUpdateTransaction:
 
     def set_admin_key(self, key: Key) -> TopicUpdateTransaction:
         """
-        Sets the public admin key for the topic.
+        Sets the admin key for the topic.
 
         Args:
-            Key: The admin key to set.
+            key: The admin key to set.
 
         Returns:
             TopicUpdateTransaction: Returns the instance for method chaining.
@@ -109,10 +112,10 @@ def set_admin_key(self, key: Key) -> TopicUpdateTransaction:
 
     def set_submit_key(self, key: Key) -> TopicUpdateTransaction:
         """
-        Sets the public submit key for the topic.
+        Sets the submit key for the topic.
 
         Args:
-            Key: The submit key to set.
+            key: The submit key to set.
 
         Returns:
             TopicUpdateTransaction: Returns the instance for method chaining.

tests/unit/topic_update_transaction_test.py

@@ -39,7 +39,12 @@ def create_key(key_type: str, use_private: bool) -> PrivateKey | PublicKey:
     Returns:
         The created key (PrivateKey or PublicKey)
     """
-    private_key = PrivateKey.generate_ed25519() if key_type == "ed25519" else PrivateKey.generate("ecdsa")
+    if key_type == "ed25519":
+        private_key = PrivateKey.generate_ed25519()
+    elif key_type == "ecdsa":
+        private_key = PrivateKey.generate("ecdsa")
+    else:
+        raise ValueError(f"Unsupported key_type: {key_type!r}")
     return private_key if use_private else private_key.public_key()
 
 
@@ -60,9 +65,32 @@ def verify_key_in_proto(proto_key, expected_public_key: PublicKey, key_type: str
     """Verify the proto key matches expected public key."""
     if key_type == "ed25519":
         assert proto_key.ed25519 == expected_public_key.to_bytes_raw()
-    else:  # ecdsa
+    elif key_type == "ecdsa":  # ecdsa
         assert proto_key.HasField("ECDSA_secp256k1")
         assert proto_key.ECDSA_secp256k1 == expected_public_key.to_bytes_raw()
+    else:
+        raise ValueError(f"Unsupported key_type: {key_type!r}")
+
+
+@pytest.mark.parametrize(
+    "key_type,use_private",
+    [
+        ("ed25519", True),
+        ("ed25519", False),
+        ("ecdsa", True),
+        ("ecdsa", False),
+    ],
+)
+def test_topic_update_setters_return_self(key_type, use_private, topic_id):
+    """Fluent setters must return self for the Key-typed API."""
+    key = create_key(key_type, use_private)
+    tx = TopicUpdateTransaction()
+
+    assert tx.set_topic_id(topic_id) is tx
+    assert tx.set_admin_key(key) is tx
+    assert tx.set_submit_key(key) is tx
+    assert tx.set_fee_schedule_key(key) is tx
+    assert tx.set_fee_exempt_keys([key]) is tx
 
 
 @pytest.mark.parametrize(