fix(promotion): handle fixed discount type in buy-get promotions

[pitzegluck]

What

Fix buy-get promotions ignoring the fixed discount type.

Why

A merchant setting up "Buy 2, get $5 off the 3rd item" gets a $5 discount treated as 5,000% off instead. The engine ignores application_method.type and always runs the percentage formula, despite the promotion model and API supporting the fixed type for buy-get promotions.

How

Branch on ApplicationMethodType.FIXED in applyPromotionToTargetItems to apply the correct calculation per type, consistent with how shipping-methods.ts handles the same logic. Rename applicablePercentage to promotionValue to match sibling file naming (shipping-methods.ts, line-items.ts). Default fallback for missing value changed from 100 to 0 to align with sibling conventions.

Testing

Added 3 integration tests: basic fixed discount, cap at item price, multi-quantity application. All 89 existing compute-actions tests pass.

Note

The JSDoc examples in service.ts for createPromotions show buyget with type: "fixed" and no value field. These examples should be updated separately.

Fixes #14934

[changeset-bot]

🦋 Changeset detected

Latest commit: ea9132a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 77 packages

Name	Type
@medusajs/promotion	Patch
@medusajs/medusa	Patch
integration-tests-http	Patch
@medusajs/test-utils	Patch
@medusajs/medusa-oas-cli	Patch
@medusajs/analytics	Patch
@medusajs/api-key	Patch
@medusajs/auth	Patch
@medusajs/caching	Patch
@medusajs/cart	Patch
@medusajs/currency	Patch
@medusajs/customer	Patch
@medusajs/file	Patch
@medusajs/fulfillment	Patch
@medusajs/index	Patch
@medusajs/inventory	Patch
@medusajs/link-modules	Patch
@medusajs/locking	Patch
@medusajs/notification	Patch
@medusajs/order	Patch
@medusajs/payment	Patch
@medusajs/pricing	Patch
@medusajs/product	Patch
@medusajs/rbac	Patch
@medusajs/region	Patch
@medusajs/sales-channel	Patch
@medusajs/settings	Patch
@medusajs/stock-location	Patch
@medusajs/store	Patch
@medusajs/tax	Patch
@medusajs/translation	Patch
@medusajs/user	Patch
@medusajs/workflow-engine-inmemory	Patch
@medusajs/workflow-engine-redis	Patch
@medusajs/draft-order	Patch
@medusajs/oas-github-ci	Patch
@medusajs/cache-inmemory	Patch
@medusajs/cache-redis	Patch
@medusajs/event-bus-local	Patch
@medusajs/event-bus-redis	Patch
@medusajs/analytics-local	Patch
@medusajs/analytics-posthog	Patch
@medusajs/auth-emailpass	Patch
@medusajs/auth-github	Patch
@medusajs/auth-google	Patch
@medusajs/caching-redis	Patch
@medusajs/file-local	Patch
@medusajs/file-s3	Patch
@medusajs/fulfillment-manual	Patch
@medusajs/locking-postgres	Patch
@medusajs/locking-redis	Patch
@medusajs/notification-local	Patch
@medusajs/notification-sendgrid	Patch
@medusajs/payment-stripe	Patch
@medusajs/core-flows	Patch
@medusajs/framework	Patch
@medusajs/js-sdk	Patch
@medusajs/modules-sdk	Patch
@medusajs/orchestration	Patch
@medusajs/types	Patch
@medusajs/utils	Patch
@medusajs/workflows-sdk	Patch
@medusajs/http-types-generator	Patch
@medusajs/cli	Patch
@medusajs/deps	Patch
@medusajs/telemetry	Patch
@medusajs/admin-bundler	Patch
@medusajs/admin-sdk	Patch
@medusajs/admin-shared	Patch
@medusajs/admin-vite-plugin	Patch
@medusajs/dashboard	Patch
@medusajs/icons	Patch
@medusajs/toolbox	Patch
@medusajs/ui-preset	Patch
create-medusa-app	Patch
medusa-dev-cli	Patch
@medusajs/ui	Patch

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[vercel]

@pitzegluck is attempting to deploy a commit to the medusajs Team on Vercel.

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[medusa-os-bot]

Thank you for your contribution! 🎉

After an initial review, this PR looks good to us. Here's a summary:

✅ PR template is complete
✅ Linked to a verified issue (#14934)
✅ Follows contribution guidelines
✅ Changeset included with correct patch bump type
✅ Fix follows the established pattern from shipping-methods.ts

Notes:

The 3 new integration tests cover allocation: "once" with fixed type for standard item promotions — which is useful coverage, but doesn't directly exercise the bug being fixed. There's no positive-case test that verifies a BUYGET promotion with type: "fixed" applies the correct dollar amount (e.g., create a promo with value: 500, satisfy the buy rules, and assert the adjustment is 500, not 50000 or 0). Adding one would specifically guard against regressions in the applyPromotionToTargetItems path.

A team member will do a final review before this is merged. We appreciate your patience!

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[shahednasser]

LGTM thanks!