Add state validation in sandbox keep alive operation

[jakubno]

Note

Keep-alive now only extends TTL for running sandboxes (returns 404 otherwise) and adds an integration test asserting 404 when setting timeout during pause.

• Backend
  • Keep-alive validation: In packages/api/internal/orchestrator/keep_alive.go, KeepAliveFor now returns NotFound if the sandbox State is not running, preventing TTL updates on non-running sandboxes.
• Tests
  • Integration: Add tests/integration/internal/tests/api/sandboxes/sandbox_set_timeout_test.go to verify setting timeout during sandbox pause returns 404.

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