Description

Per the testing conventions documented in GEMINI.md:

When testing code that depends on environment variables, use
vi.stubEnv('NAME', 'value') in beforeEach and
vi.unstubAllEnvs() in afterEach. Avoid modifying process.env
directly as it can lead to test leakage and is less reliable.

Several test files in packages/a2a-server/ still use direct
process.env manipulation:

• src/commands/init.test.ts (1 direct assignment)
• src/config/config.test.ts (~8 direct assignments/deletions)
• src/http/app.test.ts (3 direct assignments/deletions)

Proposed fix

Replace all direct process.env writes and deletes with
vi.stubEnv() / vi.unstubAllEnvs() following the pattern
documented in GEMINI.md.

I'd be happy to submit a PR for this.