feat(node): Ensure manual OTEL setup works

[mydea]

Based on the great feedback in https://github.com/getsentry/sentry-javascript/discussions/12191, this does some small adjustments to ensure that you actually can use the Node SDK properly with a custom OTEL setup:

1. Ensure we do not run validateOpenTelemetrySetup() when skipOpenTelemetrySetup is configured.

Today, this is impossible to fix because you need a client to create the sampler, and you need the sampler to satisfy the validation, but the validation runs in init(). So either you call init() before doing your manual setup, which means you get the warning, or you do the manual setup first, but then you can't actually add the sampler, and still get the warning.

This change means that users that configure skipOpenTelemetrySetup can manually call Sentry.validateOpenTelemetrySetup() if they want to get the validation, else there will be no validation automtically.

2. Export SentryContextManager from @sentry/node

This is easier to use than to use the primitive wrapContextManagerClass from @sentry/opentelemetry.

3. Ensure we always run setupEventContextTrace, not tied to skipOpenTelemetrySetup

There is no reason to tie this together, this now just always runs in init() - it's just an event processor!