Summary
This PR replaces #47656 with the correct head branch for the 6.2-dev backport flow.

It includes SMTP OAuth2 support in Joomla Global Configuration for outgoing mail, including:

New mailer option: smtpoauth2
OAuth2 token issue/check flow in Global Configuration
Provider selection:
Microsoft
Google
Custom endpoints
Microsoft tenant mode:
common endpoint
specific tenant ID endpoint
SMTP XOAUTH2 token provider integration in mail factory
Why
The previous PR #47656 used the wrong head branch (feature/oauth2-mail), which caused merge/conflict handling issues in the 6.2 backport workflow.
This PR is the clean replacement using feature/oauth2-mail-6.2.

Main changes
administrator/components/com_config/forms/application.xml
OAuth2 fields for provider/client/scope/token workflow
mailer option smtpoauth2
mailer filter fix for values ending with digits
administrator/components/com_config/src/Controller/RequestController.php
OAuth2 auth/callback/token-check flow
provider resolution and endpoint generation
administrator/components/com_config/src/Model/ApplicationModel.php
config persistence and test mail updates for OAuth2
administrator/components/com_config/src/Field/Oauth2TokenField.php
token UI actions in Global Config
PHPCS side-effects warning fix
administrator/components/com_config/src/Helper/ConfigHelper.php
callback URL helper
libraries/src/Mail/MailerFactory.php
smtpoauth2 transport wiring and XOAUTH2 setup
libraries/src/Mail/SmtpOAuth2TokenProvider.php
refresh-token based access token retrieval
administrator/language/en-GB/com_config.ini
new labels/messages
Notes
Microsoft single-tenant apps are supported through tenant-specific endpoint generation.
This PR supersedes #47656 for the 6.2-dev target.