@envelop/opentelemetry resolver spans aren’t active, nested spans attach to operation span

### Issue workflow progress

*Progress of the issue based on the**
*[*Contributor Workflow*]()

- [X] 1\. The issue provides a
[minimal reproduction]() available on
[Stackblitz]().
* *Please install the latest* `@envelop/*` packages that you are using.
* *Please make sure the reproduction is as small as possible.*
* StackBlitz: [https://stackblitz.com/github/kdawgwilk/envelop/tree/repro/opentelemetry-resolver-context/repro/opentelemetry-resolver-context?file=index.js]()
- [X] 2\. A failing test has been provided
* `packages/plugins/opentelemetry/test/use-open-telemetry.spec.ts`
- [X] 3\. A local solution has been provided
* PR: [https://github.com/graphql-hive/envelop/pull/2852]()
- [X] 4\. A pull request is pending review
* PR: [https://github.com/graphql-hive/envelop/pull/2852]()

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**Describe the bug**

When using `useOpenTelemetry({ resolvers: true })`, resolver spans are created but resolver execution is not run inside an active span context. As a result, nested spans created inside resolvers attach to the GraphQL operation span instead of the resolver span.

**To Reproduce** Steps to reproduce the behavior:

1. Enable `useOpenTelemetry` with `resolvers: true`.
2. Execute a GraphQL query whose resolvers call any instrumented library.
3. Inspect traces (or run the StackBlitz repro).

**Expected behavior**

Nested spans created during resolver execution should be children of the resolver span, not the operation span.

**Environment:**

* OS: macOS (darwin 24.6.0)
* NodeJS: v24.11.1
* `@envelop/*` versions:
* `@envelop/core`: `5.5.0`
* `@envelop/opentelemetry`: `9.1.0`
* `@envelop/on-resolve`: `7.1.0`

**Additional context**

The resolver span is created in the `useOnResolve` handler, but the resolver execution itself is not wrapped in an active span context (e.g. `context.with(trace.setSpan(...))` / `startActiveSpan`), so downstream instrumentations don’t see the resolver span as active.