feat(billing): config-gate allow_promotion_codes on subscription Checkout (#3793)

[PierreBrisorgueil]

Summary

• Adds allow_promotion_codes to the subscription Checkout session, gated on config.stripe.allowPromotionCodes === true (strict, default OFF)
• Mirrors the existing automaticTax pattern exactly (placement, style, config access path)
• Extras-pack Checkout (createExtrasCheckout) left unchanged — one-time payment, no promo-code use-case
• Fixes doc/code drift in STRIPE_SETUP.md: removes false claim that the flag was already passed unconditionally

Behavior

Config	Result
allowPromotionCodes absent	allow_promotion_codes not sent to Stripe (default)
allowPromotionCodes: false	not sent
allowPromotionCodes: 1 (truthy non-strict)	not sent — strict === true guard
allowPromotionCodes: true	allow_promotion_codes: true passed to Stripe Checkout

Downstreams opt in via config.stripe.allowPromotionCodes: true in their defaults/<project>.config.js. Existing comes/montaine/pierreb/ism are unaffected.

Test plan

• 4 new unit tests in billing.checkout.unit.tests.js: absent → undefined, false → undefined, truthy-non-strict (1) → undefined, true → present
• All 1728 unit tests pass locally (43 in billing.checkout.unit suite)
• ESLint: no issues

Closes #3793

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⚙️ Run configuration

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Run ID: d3fc79a5-6131-41b9-9508-6e9d1c55c6fc

📥 Commits

Reviewing files that changed from the base of the PR and between 428fb18 and 3aa7a99.

📒 Files selected for processing (3)

• modules/billing/STRIPE_SETUP.md
• modules/billing/services/billing.service.js
• modules/billing/tests/billing.checkout.unit.tests.js

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/billing-promo-codes-gate

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[Copilot]

Pull request overview

Adds an opt-in, strict config gate for Stripe Checkout promo codes on subscription checkouts, aligning behavior with the existing automaticTax pattern and correcting documentation drift.

Changes:

• Add checkoutParams.allow_promotion_codes = true when config?.stripe?.allowPromotionCodes === true in createCheckout.
• Add unit coverage for the new config gate (absent/false/truthy-non-true/true).
• Update modules/billing/STRIPE_SETUP.md to reflect that promo codes are config-gated (not unconditional).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
modules/billing/services/billing.service.js	Config-gates allow_promotion_codes for subscription Checkout sessions with a strict === true guard.
modules/billing/tests/billing.checkout.unit.tests.js	Adds unit tests validating the allowPromotionCodes gate behavior across key config values.
modules/billing/STRIPE_SETUP.md	Fixes doc/code drift by documenting the config-gated nature of promo codes on hosted Checkout.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.23%. Comparing base (cdacb9b) to head (3aa7a99).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3794      +/-   ##
==========================================
+ Coverage   90.19%   90.23%   +0.03%     
==========================================
  Files         152      152              
  Lines        5020     5028       +8     
  Branches     1598     1600       +2     
==========================================
+ Hits         4528     4537       +9     
+ Misses        387      386       -1     
  Partials      105      105              

Flag	Coverage Δ
integration	59.06% <0.00%> (+<0.01%)	⬆️
unit	69.64% <100.00%> (+0.30%)	⬆️

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