fix(diskqueue): use per-beat paths instead of global paths

Add a `Paths` field to diskqueue `Settings` so each beat receiver gets its own data directory for the disk queue. Thread the per-beat `*paths.Path` through the pipeline and output factory APIs:

- `outputs.Factory`, `outputs.Load`, `outputs.Success`, `outputs.SuccessNet` now accept `*paths.Path`
- `pipeline.Settings` carries `Paths` and forwards it to `queueFactoryForUserConfig`
- All 8 output factory implementations (console, discard, elasticsearch, file, kafka, logstash, redis, otelconsumer) updated to accept and forward paths
- `dockerlogbeat` updated to pass `paths.New()` explicitly. This is making explicit the previously implied behavior: the global paths were never initialized or used in `dockerlogbeat`.
- `pipeline/stress` test harness updated to use `LoadWithSettings` with paths

When `Settings.Path` is explicitly configured, it still takes precedence (backward compatible). When empty, the diskqueue directory is resolved via the per-beat `Paths.Resolve()` instead of the global `paths.Resolve()`.

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have commented my code, particularly in hard-to-understand areas
- ~~I have made corresponding changes to the documentation~~
- ~~I have made corresponding change to the default configuration files~~
- [x] I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the [`stresstest.sh`](https://github.com/elastic/beats/blob/main/script/stresstest.sh) script to run them under stress conditions and race detector to verify their stability.
- ~~I have added an entry in `./changelog/fragments` using the [changelog tool](https://github.com/elastic/elastic-agent-changelog-tool/blob/main/docs/usage.md).~~

## Disruptive User Impact

None. In the standalone beat flow, `b.Paths` and `paths.Paths` point to the same object, so behavior is identical. The change only corrects behavior for beat receivers, where each receiver gets its own paths instance.

## How to test this PR locally
### Standalone filebeat

```yaml
# filebeat-diskqueue-test.yml
path.home: /tmp/diskqueue-test/standalone
path.data: /tmp/diskqueue-test/standalone/data

filebeat.inputs:
  - type: filestream
    id: test-input
    enabled: true
    paths:
      - /tmp/diskqueue-test/input.log
    prospector.scanner.fingerprint.enabled: false
    file_identity.native: ~

queue.disk:
  max_size: 100MB

output.console:
  pretty: true
```

```bash
cd filebeat && mage build
echo '{"message": "hello"}' > /tmp/diskqueue-test/input.log
./filebeat -e -c /path/to/filebeat-diskqueue-test.yml

# Verify: diskqueue created under path.data
ls -la /tmp/diskqueue-test/standalone/data/diskqueue/
# Should contain: state.dat (and .seg files once events flow)
```

### `elastic-agent otel`
Create `/tmp/diskqueue-test/otel-receivers.yml`:

```yaml
receivers:
  filebeatreceiver/a:
    filebeat:
      inputs:
        - type: filestream
          id: stream-a
          enabled: true
          paths:
            - /tmp/diskqueue-test/input.log
          prospector.scanner.fingerprint.enabled: false
          file_identity.native: ~
    path.home: /tmp/diskqueue-test/receiver-a
    queue.disk:
      max_size: 100MB

  filebeatreceiver/b:
    filebeat:
      inputs:
        - type: filestream
          id: stream-b
          enabled: true
          paths:
            - /tmp/diskqueue-test/input.log
          prospector.scanner.fingerprint.enabled: false
          file_identity.native: ~
    path.home: /tmp/diskqueue-test/receiver-b
    queue.disk:
      max_size: 100MB

exporters:
  debug:
    verbosity: detailed

service:
  pipelines:
    logs:
      receivers: [filebeatreceiver/a, filebeatreceiver/b]
      exporters: [debug]
  telemetry:
    metrics:
      level: none
```

Run:

```bash
elastic-agent otel --config /tmp/diskqueue-test/otel-receivers.yml
```

Verify:

```bash
find /tmp/diskqueue-test/receiver-a -name state.dat
# Expected: /tmp/diskqueue-test/receiver-a/data/diskqueue/state.dat

find /tmp/diskqueue-test/receiver-b -name state.dat
# Expected: /tmp/diskqueue-test/receiver-b/data/diskqueue/state.dat
```

## Related issues

- Closes #46989

(cherry picked from commit 979f9b4)

# Conflicts:
#	libbeat/outputs/logstash/logstash.go
#	x-pack/otel/exporter/logstashexporter/exporter.go

Resolve cherry-pick conflicts from #48834 backport to 8.19:
- logstash.go: keep 8.19 code structure (no MakeLogstashClients
  refactoring), add paths import and pass beatPaths to SuccessNet
- exporter.go: remove file that doesn't exist on 8.19 (the
  logstashexporter package is a main-branch feature)