fix(rest/python): seed valid totals when creating a checkout

[vishkaty]

Problem

With the totals contains cardinality now enforced on the response models (exactly one subtotal and one total), every POST /checkout-sessions returns 500.

create_checkout constructs the checkout with an empty totals list and fills it in during _recalculate_totals (the server is the authority for pricing). Construction runs the totals validator on that empty list and raises before recalculation:

pydantic_core.ValidationError: 1 validation error for UnifiedCheckout
totals
  Value error, Array must contain at least 1 entry matching type=='subtotal' (schema minContains=1)

The final wire document is fully valid (exactly one subtotal and one total) - this is purely about when the check fires.

Fix

Seed a valid placeholder ([{subtotal, 0}, {total, 0}]) at construction so the in-progress model satisfies the constraint; _recalculate_totals overwrites it with the server-authoritative amounts immediately after. Construction stays fully validated (no model_construct, so extension fields like discounts are still coerced normally), and the emitted wire document is unchanged. Single-site change in create_checkout (the update path mutates an already-valid checkout via assignment, which is not validated).

Placeholder entries are passed as dicts so pydantic coerces them to the field's own Total type, avoiding a generated-class identity mismatch.

Verification

Booted the reference flower-shop server and ran, with the totals validators active:

	Before	After
Conformance suite	2 pass / 67 fail	69 pass / 0 fail

Also green without the validators (released SDK, as this repo's CI resolves it), confirming it is safe to land independently:

• Server integration tests (rest/python/server pytest): 16 passed
• Happy-path client (simple_happy_path_client.py): completed

ruff format --check and ruff check clean.

Relates to Universal-Commerce-Protocol/python-sdk#57 (keeps that enforcement strict while keeping the reference server green).