fix: carry buyer from checkout_obj in update_checkout_session

[donbenny0]

Description

update_checkout_session in conformance/integration_test_utils.py was not propagating
buyer information from the existing checkout object into the update payload. When no
explicit buyer argument is passed, the field stayed None and was stripped out by
exclude_none=True during serialisation.

This broke test_known_customer_one_address (and any future test relying on buyer-aware
server logic during an update), because the server received no buyer identity and could
not inject the customer's stored addresses into the fulfillment response.

Fixes: #24

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

What changed and why — quick summary for reviewers

	Before	After
currency	falls back to checkout_obj.currency	unchanged
line_items	falls back to checkout_obj.line_items	unchanged
payment	falls back to checkout_obj.payment	unchanged
buyer	always None → omitted from request	falls back to checkout_obj.buyer

The one-line fix added on line 813 of integration_test_utils.py:
integration_test_utils.py
Lines 812-813

currency = currency if currency is not None else checkout_obj.currency
buyer = buyer if buyer is not None else checkout_obj.buyer

This makes buyer consistent with every other field in the method and ensures the server always receives the customer identity during updates, enabling correct address injection for known customers.

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[donbenny0]

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[donbenny0]

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