Refactor the Create/Resume functions to avoid globals/reduce args

[djeebus]

This PR creates a Factory struct, which does a few things:

• moves networkPool and devicePool to a central place, so they don't need to be passed into CreateSandbox and ResumeSandbox on every invocation.
• moves the query for MetricsWriteFlagName into the Factory, simplifying the arguments.
• move the check of the ALLOW_SANDBOX_INTERNET env var into the main function, and out of the global scope.

---

Note

Introduce sandbox.Factory to encapsulate network/device pools and feature flags, refactoring sandbox create/resume flows and updating builders/servers to use it while removing global config and excess args.

• Core refactor:
  • Add sandbox.Factory holding network.Pool, nbd.DevicePool, featureflags.Client, and default internet access.
  • Convert sandbox.CreateSandbox/ResumeSandbox to Factory methods; eliminate global config.AllowSandboxInternet and per-call pool args.
  • Move metrics write flag resolution into Factory during ResumeSandbox.
• Server wiring:
  • server.New simplified (no context/err return); ServiceConfig gains SandboxFactory and uses it when creating sandboxes.
  • Main initializes SandboxFactory (env-driven ALLOW_SANDBOX_INTERNET) and passes it to server and template manager.
• Template build pipeline:
  • build.Builder, layer executors, and phase builders accept/use sandbox.Factory instead of pools; constructors updated accordingly.
• Cleanup:
  • Remove internal/config AllowSandboxInternet.
  • Drop redundant feature flag checks and pool args across call sites.

^{Written by Cursor Bugbot for commit 5f5caa1. This will update automatically on new commits. Configure here.}

[djeebus]

Cursor's comment is true, but intentional. It felt like the right thing to do (the CLIs are used for testing, while the main binary is deployed and configured via env vars), but if that's the wrong move, let me know.

[djeebus]

oh, the template server shouldn't have the hardcoded value, I'll fix that

[jakubno]

I think layer executor doesn't need those anymore

[djeebus]

Good catch! Removed them and a bunch of other unused fields.

[jakubno]

can you move the TODO? We're still not sending it as part of build

[jakubno]

can we keep it on separate lines? 🙏🏻 it's easier to review