Add orchestrator proxy retries to handle port forwarding delay

[Code42Cate]

Problem
When a process binds to localhost inside a sandbox, envd scans every 1s to detect it and starts socat to forward the port to the host IP (169.254.0.21). This creates a race condition where HTTP requests can arrive before forwarding is active, causing immediate connection failures.

Solution
Added retry logic in the proxy's DialContext with linear backoff:

• 5 retry attempts with backoff: 100ms, 200ms, 300ms, 400ms
• Total retry window: ~1000ms (covers envd's 1s scan interval)
• The amount of retries is configurable. Only the orchestrator proxy has 5 retries, the client proxy has 1

Behavior
Best case: Port already forwarded → immediate success (0ms)
Average: Request arrives mid-scan → 1-2 retries, ~300ms delay
Worst case (success): Request arrives just before scan → 4-5 retries, ~1000ms delay
Worst case (failure): Port doesn't exist → ~1000ms then error

Testing
Added TestProxyRetriesOnDelayedBackendStartup that simulates a 300ms backend startup delay. Test passes with retries, fails without them. You can verify that behavior by reducing the attempts to 1. The tests will then fail

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Note

Adds linear-backoff connection retries to proxy DialContext (5 for orchestrator, 1 for client) and updates pool/proxy APIs and tests, including a delayed-backend startup test.

• Proxy Core:
  • Add MaxConnectionAttempts and constants ClientProxyRetries (1) and SandboxProxyRetries (5).
  • Update reverseproxy.New(port, maxConnectionAttempts, idleTimeout, getDestination) and pass attempts to pool.
• Pool/Client:
  • Implement retry loop in pool/client.go DialContext with linear backoff (100–400ms) and context-aware exits.
  • Extend pool.New(maxClientConns, maxConnectionAttempts, idleTimeout) and propagate attempts in ProxyPool.Get to newProxyClient.
• Orchestrator/Client Proxies:
  • Wire retries: orchestrator uses SandboxProxyRetries; client uses ClientProxyRetries.
• Tests:
  • Add TestProxyRetriesOnDelayedBackendStartup to validate retry behavior.
  • Update test helpers to pass retry constant; replace several error checks with require.NoError.

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[ValentaTomas]

lgtm