Return 404 for missing simulation gateway jobs

[anth-volk]

Problem

After enabling gateway auth on beta, the auth smoke test exposed that polling a definitely missing job ID can return a 500 instead of a 404:

GET /jobs/auth-smoke-probe-does-not-exist

The endpoint expected a missing job to be reported as not found, but Modal can construct a lazy FunctionCall handle from an arbitrary object ID. That means FunctionCall.from_id(job_id) is not enough to prove the gateway issued the job or that the job exists. The endpoint then falls through into the generic worker-failure path and returns a redacted 500.

Impact

• Auth smoke tests fail even though auth enforcement itself works.
• Clients cannot distinguish a missing/unknown job from a real simulation worker failure.
• The gateway behavior diverges from Modal's documented FastAPI job queue example, which maps expired/missing output to 404.

Desired behavior

• Job IDs not issued by the gateway should return 404.
• Modal not-found or expired-output errors while polling a previously issued job should return 404.
• Running jobs should still return 202.
• Actual worker exceptions should still return redacted 500 responses.