Thanos Query crashes with "concurrent map iteration and map write" panic
in distributed mode when multiple goroutines access the same `annotations.Annotations`
map concurrently.

```
panic: concurrent map iteration and map write
github.com/prometheus/prometheus/util/annotations.(*Annotations).Merge(...)
github.com/thanos-io/promql-engine/engine.(*compatibilityQuery).Exec(...)
```

Here I replaced direct access to `res.Warnings.AsErrors()` with a thread-safe copy:
```go
// Before (unsafe)
warnings = append(warnings, res.Warnings.AsErrors()...)

// After (thread-safe)
safeWarnings := annotations.New().Merge(res.Warnings)
warnings = append(warnings, safeWarnings.AsErrors()...)
```

Signed-off-by: Joel Verezhak <jverezhak@open-systems.com>
Co-authored-by: Joel Verezhak <jverezhak@open-systems.com>
Signed-off-by: Michael Hoffmann <mhoffmann@cloudflare.com>

Signed-off-by: Michael Hoffmann <mhoffmann@cloudflare.com>