test(a2a-server): migrate process.env to vi.stubEnv() per GEMINI.md conventions

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.