test: stabilize cost discount spec

[steebchen]

Summary

• mock effective discount lookup in costs unit tests so they do not depend on seeded DB state
• preserve hardcoded provider discounts as the default mocked behavior
• explicitly assert the 10% OpenAI discount case in the discount-specific spec

Verification

• pnpm vitest run apps/gateway/src/lib/costs.spec.ts --no-file-parallelism
• pnpm format
• pnpm build

Summary by CodeRabbit

• Tests
  • Improved test coverage for discount calculations with enhanced mocking infrastructure and more comprehensive assertions to verify discount application, cost calculations, and proper system behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 4d581108-94c1-4c53-899c-6918ddd6fb4e

📥 Commits

Reviewing files that changed from the base of the PR and between d1ba037 and 6821044.

📒 Files selected for processing (1)

• apps/gateway/src/lib/costs.spec.ts

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Walkthrough

Test refactoring in costs.spec.ts to improve discount-related test coverage. Introduces centralized mock management via beforeEach hook and replaces hardcoded discount assertions with controlled mock return values.

Changes

Cohort / File(s)	Summary
Test Setup & Mock Management apps/gateway/src/lib/costs.spec.ts	Added Vitest imports (beforeEach, vi), implemented hoisted mockGetEffectiveDiscount function, added module mock for @llmgateway/db, and introduced beforeEach hook for mock reset and default behavior configuration.
Discount Test Assertion apps/gateway/src/lib/costs.spec.ts	Refactored "should apply discount when model has discount field" test to use controlled mock return values (discount: 0.1, source, discountId) and assert resulting cost calculations (discount, inputCost, outputCost, totalCost) and mock call arguments.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• fix(costs): update discount calculation to use multiplier #1048: Modifies the same discount test and changes how discount handling and assertions are performed in costs.spec.ts
• feat(models): add discount field #894: Modifies the same test file to control discount behavior through mocking getEffectiveDiscount and validating computed discounted costs

🚥 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 summarizes the main change: stabilizing/fixing flaky cost discount unit tests through mocking.
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

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-cost-discount-test

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[Copilot]

Pull request overview

Stabilizes calculateCosts unit tests by removing reliance on seeded DB discount state and explicitly controlling discount behavior via a Vitest module mock.

Changes:

• Mock @llmgateway/db#getEffectiveDiscount in costs.spec.ts using a hoisted mock to avoid module-load ordering issues.
• Provide a default mock implementation that preserves hardcoded (model/provider mapping) discounts.
• Add an explicit assertion for a 10% OpenAI discount scenario.

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[Copilot]

The test name says "when model has discount field", but the test is actually validating that a discount returned by getEffectiveDiscount (DB/global provider discount) is applied. Renaming the test (and/or adjusting the description) would avoid confusion about what behavior is under test.

Suggested change

	it("should apply discount when model has discount field", async () => {
	it("should apply discount returned by getEffectiveDiscount (e.g., global provider discount)", async () => {

[Copilot]

The default mock implementation for getEffectiveDiscount only declares 4 parameters, but the real function takes a 5th (providerModelName). It works because extra args are ignored, but matching the full signature (even if unused) will keep the mock aligned with production code and make call expectations clearer.

Suggested change

	async (_organizationId, _provider, _model, hardcodedDiscount = 0) => ({
	async (
	_organizationId,
	_provider,
	_model,
	hardcodedDiscount = 0,
	_providerModelName,
	) => ({