test(auth-server): migrate subscription route unit tests from Mocha to Jest

[vbudhram]

Because

• The auth-server is migrating from Mocha/Chai to Jest for unit tests
• 8 subscription route test files (~7,950 lines) remained on the legacy Mocha/Chai stack

This pull request

• Migrates 8 subscription route test files from Mocha/Chai to co-located Jest specs under lib/routes/subscriptions/
• Converts all Chai assertions to Jest expect() equivalents and retains sinon.assert.* for spy/stub verification
• Replaces proxyquire module stubbing with jest.mock() + jest.requireActual()
• Adds TypeScript type annotations throughout

Migration Parity Summary

File	Tests	Assertions	Verdict
apple.spec.ts	11/11	All preserved	PASS
google.spec.ts	6/6	All preserved	PASS
play-pubsub.spec.ts	6/6	All preserved	PASS
mozilla.spec.ts	18/18	All preserved	PASS
paypal.spec.ts	30/30	All preserved	PASS
paypal-notifications.spec.ts	22/22	All preserved	PASS
stripe-webhook.spec.ts	86+18 dynamic	205 preserved	PASS
stripe.spec.ts	72/72	236 preserved	PASS
Total	269+	~681	PASS

Key Translation Patterns

Mocha/Chai	Jest Equivalent
assert.deepEqual(a, b)	expect(a).toEqual(b)
assert.equal / assert.strictEqual	expect(a).toBe(b)
assert.instanceOf(x, T)	expect(x).toBeInstanceOf(T)
assert.deepInclude(a, b)	expect(a).toEqual(expect.objectContaining(b))
assert.fail('msg')	throw new Error('msg')
assert.calledOnce(spy) (merged sinon)	sinon.assert.calledOnce(spy)
proxyquire	jest.mock() + jest.requireActual()

Notable Improvements Over Original

• Fixed async describe() anti-pattern in stripe-webhook tests
• Avoided variable shadowing (error → fetchError, deleteAccountIfUnverifiedStub → localDeleteStub)
• Made implicit undefined values explicit in test data and sinon fakes

Issue

Closes: https://mozilla-hub.atlassian.net/browse/FXA-12619

Checklist

• My commit is GPG signed
• Tests pass locally (if applicable)
• Documentation updated (if applicable)
• RTL rendering verified (if UI changed)

How to review

This is a large PR so it is probably better to use some AI tools to help verify the changes. I have run analysis on the assertions and test comparions so to the best of my knowledge this has partiy with original. It would be good for others to also do the check.