bug(oauth): custom OAuth show_button=false is ignored and always saved as true

Description:

Custom OAuth show_button cannot be reliably set to false during provider initialization.
When _show_button=false is provided, the setting is still persisted as true, so the login button remains visible.

Steps to reproduce:

1. Set environment variables for a custom OAuth provider, for example:
   • Accounts_OAuth_Custom_Github=true
   • Accounts_OAuth_Custom_Github_show_button=false
2. Start Rocket.Chat.
3. Open Administration > Settings > OAuth > Custom OAuth: Github and check show_button value.
4. Check login page provider visibility.

Expected behavior:

If Accounts_OAuth_Custom_<Provider>_show_button=false is set, the provider login button should be hidden (show_button=false persisted).

Actual behavior:

show_button is persisted as true, so the provider login button is still shown.

Server Setup Information:

• Version of Rocket.Chat Server: 8.3.0-develop
• License Type: N/A
• Number of Users: N/A
• Operating System: N/A
• Deployment Method: docker/env-based setup
• Number of Running Instances: N/A
• DB Replicaset Oplog: N/A
• NodeJS Version: N/A
• MongoDB Version: N/A

Client Setup Information

• Desktop App or Browser Version: Any
• Operating System: Any

Additional context

Code path involved:

• apps/meteor/server/lib/oauth/initCustomOAuthServices.ts passes showButton from env.
• apps/meteor/server/lib/oauth/addOAuthService.ts uses values.showButton || true for Accounts_OAuth_Custom-<name>-show_button, which converts explicit false to true.

Relevant logs:

N/A

Start with apps/meteor/server/lib/oauth/initCustomOAuthServices.ts and apps/meteor/server/lib/oauth/addOAuthService.ts, tracing how the environment value reaches Accounts_OAuth_Custom--show_button. Verify that an explicit false remains false, then check the Administration OAuth setting and login-page provider visibility to confirm the reported behavior is fixed.