Fix W2-B-061: Align env tests with new env validator contract

[josephinejose-alt]

• jwt.clockskew.test.ts: Move env setup into beforeEach with jest.resetModules() to ensure env vars are set before env module loads
• env.test.ts: Remove duplicate env var assignments and use delete for missing USDC issuer tests (validator checks for undefined, not empty strings)

closes #771

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[josephinejose-alt]

@Junman140 please merge