[breaking-change] Undocumented breaking changes detected (2026-06-29)

Recent commits introduced breaking changes that appear undocumented.

## Breaking Changes

### 1. `libbeat/outputs.SuccessNet` now requires a new `worker int` argument

**Commit(s):** [`01ce4aba49059ea6ef8b7b488d786a40a34a0814`](https://github.com/elastic/beats/commit/01ce4aba49059ea6ef8b7b488d786a40a34a0814)

**PR:** https://github.com/elastic/beats/pull/51041

**What broke:** External/community Beats or custom output code importing `github.com/elastic/beats/v7/libbeat/outputs` and calling `outputs.SuccessNet` with the previous signature will no longer compile. The exported function changed from:

```go
func SuccessNet(cfg config.Namespace, loadbalance bool, batchSize, retry int, encoderFactory queue.EncoderFactory[publisher.Event], logger *logp.Logger, beatPaths *paths.Path, netclients []NetworkClient) (Group, error)
```

to requiring a new `worker int` argument before `netclients`.

**Evidence:**
- Current signature: `libbeat/outputs/util.go:93-101` now declares `func SuccessNet(... beatPaths *paths.Path, worker int, netclients []NetworkClient) (Group, error)`.
- Previous signature from the parent commit had `beatPaths *paths.Path, netclients []NetworkClient` with no `worker` argument.
- The repository’s developer changelog says it is intended for community Beat developers and covers breaking internal API changes: `CHANGELOG-developer.next.asciidoc:6-11`, including that each breaking change should explain how other Beats should migrate.
- The changelog fragment for this PR is a bug fix only and does not mention the API migration: `changelog/fragments/1780315938-fix-loadbalance-bug.yaml:12-16`.

**Why undocumented:** The PR body and user-facing docs document the `loadbalance: false` behavior change, but they do not mention the exported Go API signature break or migration guidance for community Beat/custom output callers. `CHANGELOG-developer.next.asciidoc` was not updated for this internal API break.

**Suggested fix:** Add a developer changelog or migration note explaining the new `worker` parameter and recommending callers pass the configured worker count, such as `outputs.NumofWorker(rawCfg)`, matching the updated Elasticsearch/Logstash/Redis call sites.

## Suggested Actions

- [ ] Document the `outputs.SuccessNet` signature change in `CHANGELOG-developer.next.asciidoc` or equivalent developer-facing release notes.
- [ ] Include migration guidance for custom output/community Beat callers.

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> - [x] expires on Jul 6, 2026, 2:35 PM UTC

Read CHANGELOG-developer.next.asciidoc and inspect libbeat/outputs/util.go:93-101, then compare the updated Elasticsearch, Logstash, and Redis call sites. Add a developer-facing note documenting the SuccessNet worker argument and migration guidance for community Beat or custom output callers.

Written by the indexing model from the issue text.