[bug-hunter] BeatReceiver.Start panics on Run error when beater lacks OTEL wrapper

## Impact
A receiver startup failure can crash with a panic instead of returning an error. This turns a recoverable startup error into process termination for beat receivers that do not implement `cfgfile.WithOtelFactoryWrapper`.

## Reproduction Steps
1. Create this new test file at `x-pack/libbeat/cmd/instance/repro_receiver_panic_test.go`:

```go
package instance

import (
"errors"
"testing"

"github.com/elastic/beats/v7/libbeat/beat"
beatinstance "github.com/elastic/beats/v7/libbeat/cmd/instance"
"github.com/elastic/beats/v7/libbeat/management"
"github.com/elastic/elastic-agent-libs/config"
"go.opentelemetry.io/collector/component"
"go.opentelemetry.io/collector/component/componentstatus"
)

type failingBeater struct{}

func (f *failingBeater) Run(*beat.Beat) error { return errors.New("boom") }
func (f *failingBeater) Stop() {}

type emptyHost struct{}

func (h *emptyHost) GetExtensions() map[component.ID]component.Component { return nil }
func (h *emptyHost) Report(*componentstatus.Event) {}

// Expected behavior: Start should return an error when Run fails.
// Actual behavior: Start panics because groupReporter is nil.
func TestReproStartShouldReturnErrorInsteadOfPanic(t *testing.T) {
disableMetricsLog, err := config.NewConfigFrom(map[string]interface{}{"enabled": false})
if err != nil {
t.Fatalf("config setup failed: %v", err)
}

b := &beatinstance.Beat{}
b.Manager = &management.FallbackManager{}
b.Config.MetricLogging = disableMetricsLog

br := &BeatReceiver{beat: b, beater: &failingBeater{}}

if err := br.Start(&emptyHost{}); err == nil {
t.Fatal("expected run error, got nil")
}
}
```

2. Run:

```bash
go test ./x-pack/libbeat/cmd/instance -run TestReproStartShouldReturnErrorInsteadOfPanic -count=1
```

## Expected vs Actual
**Expected:** `BeatReceiver.Start` returns `beat receiver run error: boom` (or equivalent wrapped error).

**Actual:** panic due to nil dereference:

```text
--- FAIL: TestReproStartShouldReturnErrorInsteadOfPanic (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
...
github.com/elastic/beats/v7/x-pack/libbeat/cmd/instance.(*BeatReceiver).Start(...)
/home/runner/work/beats/beats/x-pack/libbeat/cmd/instance/receiver.go:187 +0x5ef
...
FAIL github.com/elastic/beats/v7/x-pack/libbeat/cmd/instance 0.022s
```

## Failing Test
The full failing test is included in Reproduction Step 1.

## Evidence
- `x-pack/libbeat/cmd/instance/receiver.go:119-122` initializes `groupReporter` only if `br.beater` implements `cfgfile.WithOtelFactoryWrapper`.
- `x-pack/libbeat/cmd/instance/receiver.go:185-188` unconditionally calls `groupReporter.UpdateStatus(...)` on run error.
- `git blame` shows line 187 (`groupReporter.UpdateStatus`) was added in commit `2ac081b1086`, making the nil call reachable when `Run` returns an error and wrapper is not implemented.

---
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> - [x] expires on Mar 24, 2026, 12:12 PM UTC

Start with x-pack/libbeat/cmd/instance/receiver.go, especially the groupReporter initialization at lines 119-122 and the run-error path at lines 185-188. Add the regression test from the issue as x-pack/libbeat/cmd/instance/repro_receiver_panic_test.go, then run go test ./x-pack/libbeat/cmd/instance -run TestReproStartShouldReturnErrorInsteadOfPanic -count=1. Done means Start returns the run error instead of panicking when the beater lacks the OTEL wrapper.

Written by the indexing model from the issue text.