Allow searching issues by ID (#31479)

When you are entering a number in the issue search, you likely want the
issue with the given ID (code internal concept: issue index).
As such, when a number is detected, the issue with the corresponding ID
will now be added to the results.

Fixes #4479

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/indexer/issues/internal/model.go

@@ -4,6 +4,8 @@
 package internal
 
 import (
+	"strconv"
+
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/modules/optional"
 	"code.gitea.io/gitea/modules/timeutil"
@@ -124,6 +126,12 @@ func (o *SearchOptions) Copy(edit ...func(options *SearchOptions)) *SearchOption
 	return &v
 }
 
+// used for optimized issue index based search
+func (o *SearchOptions) IsKeywordNumeric() bool {
+	_, err := strconv.Atoi(o.Keyword)
+	return err == nil
+}
+
 type SortBy string
 
 const (