feat: Ability to specify the authentication type for imap server (#12306)

# Pull Request Template

## Description

This PR adds IMAP authentication mechanism selection to Chatwoot's email
inbox configuration. Users can now choose between 'plain', 'login', and
'cram-md5' authentication methods when configuring IMAP settings,
providing flexibility for different email providers that require
specific authentication types.

https://github.com/chatwoot/chatwoot/issues/8867

The implementation includes:
- Frontend dropdown with numeric keys (1, 2, 3) matching SMTP auth style
- Backend API validation for allowed authentication mechanisms
- Consistent 'cram-md5' format throughout the codebase
- Updated IMAP service to handle different auth types properly

This feature maintains consistency with existing SMTP authentication
options and follows the established UI/UX patterns in the application.

## Type of change

Please delete options that are not relevant.

- [x] New feature (non-breaking change which adds functionality)
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

### Manual Testing:
- Tested in Docker environment
- Verified IMAP auth dropdown appears in inbox settings
- Confirmed all three auth mechanisms (plain, login, cram-md5) can be
selected and saved
- Tested API validation by attempting to save invalid auth mechanisms

### Automated Testing:
- Updated existing IMAP service tests to use consistent lowercase values
- Updated API controller tests for authentication parameter handling
- All tests pass locally with the new changes

### Test Configuration:
- Tested with both new and existing inbox configurations

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] Any dependent changes have been merged and published in downstream
modules

## Additional Notes

- This feature is backward compatible and doesn't break existing IMAP
configurations
- The 'cram-md5' format is used consistently throughout (UI, API,
storage, services)
- Net::IMAP compatibility is maintained by converting to 'CRAM-MD5'
internally
- Follows the same pattern established by SMTP authentication
configuration

---------

Co-authored-by: João Santos <joao.santos@madigital.eu>
Co-authored-by: Sony Mathew <sony@chatwoot.com>

spec/controllers/api/v1/accounts/inboxes_controller_spec.rb

@@ -568,8 +568,10 @@ RSpec.describe 'Inboxes API', type: :request do
         email_channel = create(:channel_email, account: account)
         email_inbox = create(:inbox, channel: email_channel, account: account)
 
-        imap_connection = double
-        allow(Mail).to receive(:connection).and_return(imap_connection)
+        imap_connection = instance_double(Net::IMAP, disconnected?: false)
+        allow(Net::IMAP).to receive(:new).and_return(imap_connection)
+        allow(imap_connection).to receive(:login)
+        allow(imap_connection).to receive(:disconnect)
 
         patch "/api/v1/accounts/#{account.id}/inboxes/#{email_inbox.id}",
               headers: admin.create_new_auth_token,
@@ -578,7 +580,8 @@ RSpec.describe 'Inboxes API', type: :request do
                   imap_enabled: true,
                   imap_address: 'imap.gmail.com',
                   imap_port: 993,
-                  imap_login: 'imaptest@gmail.com'
+                  imap_login: 'imaptest@gmail.com',
+                  imap_authentication: 'login'
                 }
               },
               as: :json
@@ -587,6 +590,7 @@ RSpec.describe 'Inboxes API', type: :request do
         expect(email_channel.reload.imap_enabled).to be true
         expect(email_channel.reload.imap_address).to eq('imap.gmail.com')
         expect(email_channel.reload.imap_port).to eq(993)
+        expect(email_channel.reload.imap_authentication).to eq('login')
       end
 
       it 'updates avatar when administrator' do