Thread Safe mode (ZTS) support

**Is your feature request related to a problem? Please describe.**
Testing the OpenTelemetry agent with FrankenPHP server which runs PHP in ZTS mode yields the extensions unable to load:

```log
INFO [elastic_otel_loader][PID: 137][TID: 150] Thread Safe mode (ZTS) is not supported by Elastic Distribution for OpenTelemetry PHP.
```

**Describe the solution you'd like**
Adding support for Thread Safe mode (ZTS)

**Describe alternatives you've considered**
The only alternative I can think of is building a custom FrankenPHP version with APM agent extension compiled In, which is not viable due to:
- Requires complex multi-stage Docker build
- Significant build time
- High maintenance burden for updates
- Build complexity makes debugging difficult

Thanks for the awesome work your team is doing

Start by locating the loader's Thread Safe mode (ZTS) rejection and the extension loading checks involved when running under FrankenPHP. Run the existing agent tests in a ZTS-enabled PHP environment, then verify that the extensions load successfully and that the rejection is covered by appropriate tests.

Written by the indexing model from the issue text.