MM-56906: Remove redundant calls on team switch (#30771)

On page load, we load ALL channels and channel members from all teams.
But then, on team_switch, we would again load channels and channel
members from that team. This was redundant and mainly kept
because previously the websocket events were considered unreliable.

Now with reliable websockets, and client-side pings, we can detect
broken connections faster and recover without loss.

Additionally, the getAllChannelMembers call would page through
all responses on the client side. This was inefficient and incur
extra latency. To optimize for this, we introduce server-side
streaming of the full response if page is set to -1.

This optimizes the intial response as well.

https://mattermost.atlassian.net/browse/MM-56906

```release-note
Optimize team switch operation by removing calls to get channels
and channel members.
```


Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/channel_member.go

@@ -108,6 +108,12 @@ type ChannelMemberForExport struct {
 	Username    string
 }
 
+type ChannelMemberCursor struct {
+	Page          int // If page is -1, then FromChannelID is used as a cursor.
+	PerPage       int
+	FromChannelID string
+}
+
 func (o *ChannelMember) IsValid() *AppError {
 	if !IsValidId(o.ChannelId) {
 		return NewAppError("ChannelMember.IsValid", "model.channel_member.is_valid.channel_id.app_error", nil, "", http.StatusBadRequest)

server/public/model/client4.go

@@ -4,6 +4,7 @@
 package model
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"encoding/json"
@@ -3696,6 +3697,37 @@ func (c *Client4) GetChannelMembersWithTeamData(ctx context.Context, userID stri
 	defer closeBody(r)
 
 	var ch ChannelMembersWithTeamData
+
+	// Check if we need to handle NDJSON format (when page is -1)
+	if page == -1 {
+		// Process NDJSON format (each JSON object on new line)
+		contentType := r.Header.Get("Content-Type")
+		if contentType == "application/x-ndjson" {
+			scanner := bufio.NewScanner(r.Body)
+			ch = ChannelMembersWithTeamData{}
+
+			for scanner.Scan() {
+				line := scanner.Text()
+				if line == "" {
+					continue
+				}
+
+				var member ChannelMemberWithTeamData
+				if err2 := json.Unmarshal([]byte(line), &member); err2 != nil {
+					return nil, BuildResponse(r), NewAppError("GetChannelMembersWithTeamData", "api.marshal_error", nil, "", http.StatusInternalServerError).Wrap(err2)
+				}
+				ch = append(ch, member)
+			}
+
+			if err2 := scanner.Err(); err2 != nil {
+				return nil, BuildResponse(r), NewAppError("GetChannelMembersWithTeamData", "api.marshal_error", nil, "", http.StatusInternalServerError).Wrap(err2)
+			}
+
+			return ch, BuildResponse(r), nil
+		}
+	}
+
+	// Standard JSON format
 	err = json.NewDecoder(r.Body).Decode(&ch)
 	if err != nil {
 		return nil, BuildResponse(r), NewAppError("GetChannelMembersWithTeamData", "api.marshal_error", nil, "", http.StatusInternalServerError).Wrap(err)