[MM-54730] Don't use old hardcoded rule for validating imported posts (#25823)

* don't use old hardcoded rule for validating imported posts

* fix http verb in doc

* Use client config

* Handle local mode

* E2E tests

* Enforce default if unable to use real limit

* Unit tests

* Fix tests

* Use model.PostMessageMaxRunesV2 as lower default

* Update direct post message length validation

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: streamer45 <cstcld91@gmail.com>

server/cmd/mmctl/commands/importer/validate.go

@@ -68,6 +68,8 @@ type Validator struct { //nolint:govet
 	directPosts    uint64
 	emojis         map[string]ImportFileInfo
 
+	maxPostSize int
+
 	start time.Time
 	end   time.Time
 
@@ -96,6 +98,7 @@ func NewValidator(
 	serverChannels map[ChannelTeam]*model.Channel,
 	serverUsers map[string]*model.User,
 	serverEmails map[string]*model.User,
+	maxPostSize int,
 ) *Validator {
 	v := &Validator{
 		archiveName:           name,
@@ -118,6 +121,8 @@ func NewValidator(
 		channels: map[ChannelTeam]ImportFileInfo{},
 		users:    map[string]ImportFileInfo{},
 		emojis:   map[string]ImportFileInfo{},
+
+		maxPostSize: maxPostSize,
 	}
 
 	v.loadFromServer()
@@ -722,7 +727,7 @@ func (v *Validator) validateUser(info ImportFileInfo, line imports.LineImportDat
 
 func (v *Validator) validatePost(info ImportFileInfo, line imports.LineImportData) (err error) {
 	ivErr := validateNotNil(info, "post", line.Post, func(data imports.PostImportData) *ImportValidationError {
-		appErr := imports.ValidatePostImportData(&data, model.PostMessageMaxRunesV1)
+		appErr := imports.ValidatePostImportData(&data, v.maxPostSize)
 		if appErr != nil {
 			return &ImportValidationError{
 				ImportFileInfo: info,
@@ -854,7 +859,7 @@ func (v *Validator) validateDirectChannel(info ImportFileInfo, line imports.Line
 
 func (v *Validator) validateDirectPost(info ImportFileInfo, line imports.LineImportData) (err error) {
 	ivErr := validateNotNil(info, "direct_post", line.DirectPost, func(data imports.DirectPostImportData) *ImportValidationError {
-		appErr := imports.ValidateDirectPostImportData(&data, model.PostMessageMaxRunesV1)
+		appErr := imports.ValidateDirectPostImportData(&data, v.maxPostSize)
 		if appErr != nil {
 			return &ImportValidationError{
 				ImportFileInfo: info,