feat(inventory): add Inventory endpoint family (#242)

* feat(inventory): add Inventory endpoint family

Implements the new agentic:inventory OAuth-scoped endpoints: stock
adjustments, atomic multi-variant reservations (create/get/commit/release),
stock levels (get/set), and beta product knowledge (get/set/delete).

Error responses surface through the SDK's existing CheckoutApiException,
documented in method docstrings rather than a dedicated error class.

* refactor(inventory): dedupe shared product-knowledge fields

Extract the ~30 fields identical between InventoryProductKnowledge and
InventorySetProductRequest into an InventoryMerchandisingFields base class.
Fixes the SonarCloud duplication gate (32.4% on new code) without changing
the serialized shape of either class.

Author: armando-rodriguez-cko

checkout_sdk/checkout_api.py

@@ -11,6 +11,7 @@
 from checkout_sdk.forex.forex_client import ForexClient
 from checkout_sdk.checkout_apm_api import CheckoutApmApi
 from checkout_sdk.instruments.instruments_client import InstrumentsClient
+from checkout_sdk.inventory.inventory_client import InventoryClient
 from checkout_sdk.issuing.issuing_client import IssuingClient
 from checkout_sdk.payments.contexts.contexts_client import PaymentContextsClient
 from checkout_sdk.payments.sessions.sessions_client import PaymentSessionsClient
@@ -105,6 +106,7 @@ def __init__(self, configuration: CheckoutConfiguration):
         self.forward = ForwardClient(api_client=forward_api_client, configuration=configuration)
         self.setups = PaymentSetupsClient(api_client=base_api_client, configuration=configuration)
         self.agentic_commerce = AgenticCommerceClient(api_client=base_api_client, configuration=configuration)
+        self.inventory = InventoryClient(api_client=base_api_client, configuration=configuration)
         self.apple_pay = ApplePayClient(api_client=base_api_client, configuration=configuration)
         self.google_pay = GooglePayClient(api_client=base_api_client, configuration=configuration)
         self.standalone_account_updater = StandaloneAccountUpdaterClient(api_client=base_api_client,