feat(pr create, issue create): search-based assignee selection in MetadataSurvey

Wire up MultiSelectWithSearch for assignees in MetadataSurvey, replacing
the static MultiSelect that required bulk fetching all assignable actors.
This applies to both gh pr create and gh issue create interactive flows
when selecting assignees via the 'Add metadata' prompt.

Changes:
- Add assigneeSearchFunc parameter to MetadataSurvey
- Skip assignee bulk fetch when search func is available
- New SearchRepoAssignableActors API function for repo-level search
  (create flows have no issue/PR node ID yet)
- New RepoAssigneeSearchFunc in shared editable.go
- Refactor actorsToSearchResult helper shared by both search functions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

api/queries_repo.go

@@ -1298,6 +1298,69 @@ func RepoAssignableActors(client *Client, repo ghrepo.Interface) ([]AssignableAc
 	return actors, nil
 }
 
+// SearchRepoAssignableActors searches assignable actors for a repository with an optional
+// query string. Unlike RepoAssignableActors which fetches all actors with pagination, this
+// returns up to 10 results matching the query, suitable for search-based selection.
+func SearchRepoAssignableActors(client *Client, repo ghrepo.Interface, query string) ([]AssignableActor, int, error) {
+	type responseData struct {
+		Repository struct {
+			AssignableUsers struct {
+				TotalCount int
+			}
+			SuggestedActors struct {
+				Nodes []struct {
+					User struct {
+						ID       string
+						Login    string
+						Name     string
+						TypeName string `graphql:"__typename"`
+					} `graphql:"... on User"`
+					Bot struct {
+						ID       string
+						Login    string
+						TypeName string `graphql:"__typename"`
+					} `graphql:"... on Bot"`
+				}
+			} `graphql:"suggestedActors(first: 10, query: $query, capabilities: CAN_BE_ASSIGNED)"`
+		} `graphql:"repository(owner: $owner, name: $name)"`
+	}
+
+	var q *githubv4.String
+	if query != "" {
+		v := githubv4.String(query)
+		q = &v
+	}
+
+	variables := map[string]interface{}{
+		"owner": githubv4.String(repo.RepoOwner()),
+		"name":  githubv4.String(repo.RepoName()),
+		"query": q,
+	}
+
+	var result responseData
+	if err := client.Query(repo.RepoHost(), "SearchRepoAssignableActors", &result, variables); err != nil {
+		return nil, 0, err
+	}
+
+	var actors []AssignableActor
+	for _, node := range result.Repository.SuggestedActors.Nodes {
+		if node.User.TypeName == "User" {
+			actors = append(actors, AssignableUser{
+				id:    node.User.ID,
+				login: node.User.Login,
+				name:  node.User.Name,
+			})
+		} else if node.Bot.TypeName == "Bot" {
+			actors = append(actors, AssignableBot{
+				id:    node.Bot.ID,
+				login: node.Bot.Login,
+			})
+		}
+	}
+
+	return actors, result.Repository.AssignableUsers.TotalCount, nil
+}
+
 type RepoLabel struct {
 	ID   string
 	Name string