fix: suppress false-positive error log when PR is already in the automerge queue (#9784)

Prevent an error from being logged when a PR is added to the automerge queue and already present; I get this error a few times a day in my logs from routine Renovate activity:
```
Error adding pullID: 1206 to the automerge queue already in queue
```

Exemption for this error is commonly used in other places where the queue infrastructure is in-use, such as:
https://codeberg.org/forgejo/forgejo/src/commit/434f9465908e2c735c1bc63b9461473c35254d22/modules/indexer/issues/util.go#L178-L179

Testing: Not sure the exact activity that causes this error, and so I haven't been able to reproduce it.  The change is low risk, but I've performed no testing (other than build & lint).  Will follow-up with more detailed work if the error continues after this change.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9784
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

services/shared/automerge/automerge.go

@@ -5,6 +5,7 @@ package automerge
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"strconv"
 	"strings"
@@ -22,7 +23,7 @@ var PRAutoMergeQueue *queue.WorkerPoolQueue[string]
 
 func addToQueue(pr *issues_model.PullRequest, sha string) {
 	log.Trace("Adding pullID: %d to the automerge queue with sha %s", pr.ID, sha)
-	if err := PRAutoMergeQueue.Push(fmt.Sprintf("%d_%s", pr.ID, sha)); err != nil {
+	if err := PRAutoMergeQueue.Push(fmt.Sprintf("%d_%s", pr.ID, sha)); err != nil && !errors.Is(err, queue.ErrAlreadyInQueue) {
 		log.Error("Error adding pullID: %d to the automerge queue %v", pr.ID, err)
 	}
 }