WooCommerce connector: add order import fixture

[sridharkalaibala]

Parent issue: #34

Related: #72, #82

Goal

Add fixtures/tests for importing WooCommerce orders into the Posnic sales contract safely and idempotently.

Suggested files to inspect

• api/tests/
• sales idempotency contract docs
• Issue API v1: document sales import idempotency contract #72
• Issue WooCommerce connector: document setup and API assumptions #82
• Issue App and connector platform: reviewable integrations without weakening the offline POS core #34

Acceptance criteria

• Fixture imports a paid order once and rejects/ignores the duplicate retry.
• Fixture covers COD/unpaid, cancelled, refunded, unknown SKU, and tax/discount lines.
• External order id is preserved for audit and duplicate prevention.
• No live WooCommerce network call is made.
• The PR links back to App and connector platform: reviewable integrations without weakening the offline POS core #34.

Validation

• Run focused order import tests.
• Run git diff --check.

Out of scope

• Real WooCommerce credentials.
• Changing sale totals without tests.