spec: mark SOCKS5 egress proxy as x-not-implemented (#3382)

Adds the `x-not-implemented: true` vendor extension to the SOCKS5 egress
proxy parts of the OpenAPI spec: the `SandboxEgressProxyConfig`
component schema and the `egressProxy` properties in
`SandboxNetworkConfig` and `SandboxNetworkUpdateConfig`. This lets spec
consumers (docs, SDK generators) detect that the feature is not
implemented yet. The only Go change is the regenerated embedded spec in
`packages/api/internal/api/api.gen.go`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mishushakov