FUEL-1756 Add discount to purchase event

[dongfueled]

Summary by CodeRabbit

• New Features
  • Purchase tracking now includes discount amounts in analytics data, improving visibility into order totals and promotional impact.
  • Reports and integrations that consume purchase events will reflect discounts, helping with revenue attribution and marketing analysis.

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[coderabbitai]

Walkthrough

A "discount" field was added to the Facebook Pixel Purchase payload returned by format(), set to Number(checkoutData.discountsAmount?.amount ?? 0) when present or 0 otherwise.

Changes

Cohort / File(s)	Summary
Facebook Pixel Purchase Payload shopify/purchase_custom_pixel.js	Added discount numeric field to the object returned by format(), sourced from checkoutData.discountsAmount.amount with a 0 fallback.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Possibly related PRs

• add purchase_custom_pixel #17: Modifies the same shopify/purchase_custom_pixel.js purchase payload construction.

Suggested reviewers

• aklishchevskiy

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main change: adding a discount field to the purchase event payload.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch FUEL-1756

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@shopify/purchase_custom_pixel.js`:
- Line 62: The discount field may end up as a string; coerce it to a numeric
type when building the payload by wrapping the entire expression in Number with
a safe fallback, e.g. replace discount: checkoutData.discountsAmount?.amount ??
0 with discount: Number(checkoutData.discountsAmount?.amount ?? 0) so the
discount value is always a Number (consistent with value which is already cast).