feat(plugin-multi-tenant): add tenantSelectorComponent option to cust…

[Lezzio]

What?

• Add tenantSelectorComponent option
  Introduces a new tenantSelectorComponent?: CustomComponent | null field on the multi-tenant plugin config, with three modes:
  1. Custom: supply your own component → rendered in the admin UI
  2. Disabled: supply null → no selector is rendered
  3. Default: leave it undefined → the existing default selector is used

Why?

• Enable multi-tenant selector UI and UX flexibility
  Developers can now completely disable the selector or swap in a custom implementation.

Examples:

• This is useful when willing to customise and workaround the UX problem described in the issue Switching tenant while editing a document unintentionally changes document tenant #12369.
  In this issue switching tenants via the selector edits the current document’s tenant field, causing unintentional cross-tenant migrations. A custom tenant selector allowed me and others to customise the logic and redirect to the home page before mutating the tenant field (which avoids the document field unintentional modification).

• Some developers just want to opt it out (no needs for the selector) Multi Tenant Plugin | Disable the Tenant Selector Provider + Dropdown #11856

This property is an intuitive addition in the current design and it unlocks customization and workarounds for UI and UX

How?

1. Schema change

   /**
    * Override or disable the TenantSelector shown above the nav.
    *
    * - supply a `CustomComponent` → your component is rendered
    * - supply `null`              → no selector is rendered
    * - leave it `undefined`       → the default selector is rendered
    *
    * @default undefined
    */
   tenantSelectorComponent?: CustomComponent | null

2. Component setup

   /**
    * Add tenant selector to admin UI
    */
   const defaultTenantSelector: CustomComponent = {
     clientProps: { label: tenantSelectorLabel },
     path: '@payloadcms/plugin-multi-tenant/client#TenantSelector',
   }
   
   if (pluginConfig.tenantSelectorComponent !== null) {
     incomingConfig.admin.components.beforeNavLinks.push(
       pluginConfig.tenantSelectorComponent ?? defaultTenantSelector,
     )
   }

3. Link to issue
   Provides a configuration-level workaround for the behavior detailed in [#12369](Switching tenant while editing a document unintentionally changes document tenant #12369) without baking in a specific redirect or mutation strategy.