collector: add reusable runtime wrapper

[nicolastakashi]

Summary

This continues the incremental refactor to make node_exporter easier to embed outside the standalone binary.
The broader goal is to support ongoing work to make Prometheus exporters embeddable inside the Prometheus OpenTelemetry Collector distribution as native receivers, without requiring each exporter to be rewritten as an OpenTelemetry component.
This PR builds on the reusable config package by adding a runtime layer that downstream callers can construct directly from config, independent of CLI parsing and HTTP serving.
Specifically, this PR:

• adds collector.Runtime as a reusable wrapper around NodeCollector
• validates reusable config at runtime construction time
• exposes embeddable APIs for:
  • collector access via Collectors()
  • registry creation via Registry()
  • enabled collector discovery via EnabledCollectors()
• adds unit tests covering runtime construction, validation, and registry gathering

Why

Today, node_exporter collector construction is tied closely to the binary wiring. Introducing a reusable runtime layer is a small but important step toward separating:

• binary concerns such as flags, HTTP serving, and process setup
• reusable exporter concerns such as config validation, collector construction, and metric registry assembly
  This does not make node_exporter fully embeddable yet. In particular, collector-specific process-global configuration and state still need follow-up refactors. The intent here is to establish a reusable runtime API that future changes can build on.

Stack

This PR is part of a stacked series:

• depends on config: add reusable exporter config package #3660
  Only the runtime layer is intended to be reviewed here; the CLI wiring changes will come in a follow-up PR.