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/internal/elasticsearch/indexer_test.go

@@ -0,0 +1,44 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package elasticsearch
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func newRealIndexer(t *testing.T) *Indexer {
+	t.Helper()
+	url := "http://elasticsearch:9200"
+	if os.Getenv("CI") == "" {
+		url = os.Getenv("TEST_ELASTICSEARCH_URL")
+		if url == "" {
+			t.Skip("TEST_ELASTICSEARCH_URL not set and not running in CI")
+		}
+	}
+	indexName := "gitea_test_" + strings.ReplaceAll(strings.ToLower(t.Name()), "/", "_")
+	ix := NewIndexer(url, indexName, 1, `{"mappings":{"properties":{"x":{"type":"keyword"}}}}`)
+	_, err := ix.Init(t.Context())
+	require.NoError(t, err)
+	t.Cleanup(ix.Close)
+	return ix
+}
+
+func TestPing(t *testing.T) {
+	ix := newRealIndexer(t)
+	require.NoError(t, ix.Ping(t.Context()))
+}
+
+func TestDeleteSwallows404(t *testing.T) {
+	ix := newRealIndexer(t)
+	require.NoError(t, ix.Delete(t.Context(), "missing-id"))
+}
+
+func TestBulkAcceptsDelete404(t *testing.T) {
+	ix := newRealIndexer(t)
+	require.NoError(t, ix.Bulk(t.Context(), []BulkOp{DeleteOp("missing-id")}))
+}