Fix exec stream: remove stdin from Exec URL and StreamWithContext

[azemoning]

Problem

The exec stream wasn't producing any data. When spe created an ephemeral container and exec'd tcpdump into it, the pcap file was always 0 bytes, even though tcpdump was running and traffic was flowing.

Root cause

The Exec method had stdin=true in the URL parameters. When StreamWithContext was called with io.NopCloser(bytes.NewReader(nil)) as stdin, the SPDY connection received EOF on stdin immediately, which caused the connection to close before any stdout data could flow.

Fix

• Removed stdin=true from the Exec URL parameters in internal/k8s/client.go
• Removed Stdin from the StreamWithContext call in internal/capture/ephemeral.go
• Increased ephemeral container wait timeout from 30s to 120s for clusters that need to pull the debug image

Testing

Verified on kind cluster:

• Single pod capture: 230 bytes captured (was 0 bytes before fix)
• Inventory traffic: 2552 bytes captured
• Payment traffic: 2496 bytes captured
• DNS traffic: 895 bytes captured