Cleanup connections handling

[ValentaTomas]

Cleanup connections handling in the orchestrator proxy and add more logs to the template builder to catch connection problems.

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Note

Adds a keep-alives toggle to proxy/pool (disabled for orchestrator), standardizes sandbox HTTP transport/client, introduces a constant client proxy connection key, improves logging/metrics, and resets connections during template operations.

• Proxy/Pool:
  • Add disableKeepAlives option across proxy.New and pool/clients; set to true for orchestrator proxy and false for client proxy.
  • Set ForceAttemptHTTP2=false when creating transports.
  • Introduce pool.ClientProxyConnectionKey and use it in client-proxy.
  • Include execution.id in proxy pool logs; debug log on client creation.
• Sandbox HTTP client:
  • Add SandboxHttpTransport (keep-alives disabled) and sandboxHttpClient; replace direct httpClient usage in envd.go, health.go, and metrics.go.
  • Use SandboxHttpTransport in template build tools (sandboxtools/command.go, sandboxtools/file.go); drain response body on upload error.
• Orchestrator Template Builder:
  • Remove proxy connections from pool after restarting envd and on cleanup; add debug/error logs.
• Metrics:
  • Client proxy: add observers for pool size and server connections.
• Tests:
  • Update proxy constructor calls to pass keep-alives flag.

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