fix(plugin-multi-tenant): respect undefined initialValue and userHasAccessToAllTenants on single-tenant auto-select (#17833)

[Rodrigoue9]

📌 Summary

Fixes #17833

Fixes an issue in @payloadcms/plugin-multi-tenant where single-tenant auto-selection overrode the server's intentional "no tenant selected" state on every page mount for users with userHasAccessToAllTenants.

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🔍 Root Cause Analysis

When an installation had exactly one tenant, TenantSelectionProvider unconditionally auto-selected the first tenant and wrote the payload-tenant cookie, ignoring whether the user had global access (userHasAccessToAllTenants) and desired an unselected platform-wide view. On the client, the sync effect and setTenant also forced re-selection of the single tenant even when clearing the selection or when initialValue was undefined.

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🛠️ Solution

• In TenantSelectionProvider (server component), checked userHasAccessToAllTenants(user) before falling back to auto-selecting the single tenant, ensuring users with global access retain initialValue = undefined when no tenant cookie is set.
• In TenantSelectionProviderClient, only auto-select the single tenant and set the cookie if initialValue is defined, and allow setTenant({ id: undefined }) to clear tenant selection regardless of tenant count.
• Added unit tests in packages/plugin-multi-tenant/src/providers/TenantSelectionProvider/tenantSelection.unit.test.ts.