Replace olivere/elastic with REST API client, add OpenSearch support (#37411)

Drops `github.com/olivere/elastic/v7` (unmaintained) and replaces it
with a small in-house wrapper that speaks the Elasticsearch REST API
directly via `net/http`. The subset used by Gitea (`_cluster/health`,
`_bulk`, `_doc`, `_delete_by_query`, `_refresh`, `_search`, `HEAD`/`PUT`
index) is stable across the targeted servers, so no client library is
needed.

**Targets tested**
- Elasticsearch 7, 8, 9
- OpenSearch 1, 2, 3

**Why not `go-elasticsearch`?**
The official client enforces an `X-Elastic-Product` server-identity
check that OpenSearch deliberately fails, which would force shipping a
transport shim to defeat it. Going direct over `net/http` removes that
fight along with several MB of transitive deps (`elastic-transport-go`,
`go.opentelemetry.io/otel{,/metric,/trace}`, `auto/sdk`, `easyjson`,
`intern`, `logr`, `stdr`).

Replaces: #30755
Fixes: https://github.com/go-gitea/gitea/issues/30752

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/indexer/code/indexer_test.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"slices"
 	"testing"
+	"time"
 
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/models/unittest"
@@ -39,6 +40,16 @@ func TestMain(m *testing.M) {
 func testIndexer(name string, t *testing.T, indexer internal.Indexer) {
 	t.Run(name, func(t *testing.T) {
 		assert.NoError(t, setupRepositoryIndexes(t.Context(), indexer))
+		// Wait for the index to catch up: ES/OpenSearch make writes visible
+		// only after a refresh (default interval: 1s). Bleve is synchronous
+		// and passes on the first iteration.
+		require.Eventually(t, func() bool {
+			total, _, _, err := indexer.Search(t.Context(), &internal.SearchOptions{
+				Keyword:   "Description",
+				Paginator: &db.ListOptions{Page: 1, PageSize: 1},
+			})
+			return err == nil && total > 0
+		}, 10*time.Second, 100*time.Millisecond, "index did not become searchable")
 
 		keywords := []struct {
 			RepoIDs    []int64