Fix CI fixture process shutdown and HTTP isolation

[leehack]

Summary

• Explicitly exit the code-zero request fixture without waiting for stdin cancellation on Windows.
• Allocate a fresh HTTP fixture port per test and await server termination to avoid observing a previous server during startup.
• No SDK runtime changes.

Validation

• 53 stdio and TypeScript interoperability tests passed locally.
• HTTP tests passed in five repeated runs.
• Dart analysis, formatting, and git diff checks passed.
• Windows validation remains dependent on CI.