fix: shutdown drain can hang when downloadStream exits without response send

[biodrone]

Summary

• Replace the shutdown response channel drain with a downloadWg WaitGroup, matching the existing vodWg / postScriptWg pattern
• Remove the unused response parameter and SIGINT-only sends from downloadStream
• Add a unit test documenting the WaitGroup shutdown pattern

Fixes #583

Test plan

• go test ./...
• Send SIGINT while multiple live downloads are active and confirm the process exits promptly
• Send SIGINT while one stream ends naturally at the same moment and confirm no hang

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b1b1dec7-b9e8-4592-b5e9-d7a5b79a5042

📥 Commits

Reviewing files that changed from the base of the PR and between f499951 and 5bb2704.

📒 Files selected for processing (4)

• download_stream.go
• shutdown_test.go
• streamdl.go
• tests/integration/.gitignore

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/shutdown-drain-hang-583

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