feat: add tenant selection store (TenantStore)

[RawanMatar89]

Summary

Adds TenantStore: single source of truth for the selected tenant and the tenant catalog, persisted to UserDefaults, posting .tenantDidChange for observers. Still foundation — nothing in the app resolves or calls it yet.

Included

• TenantStore (Core): selection state + catalog, auto-selects the sole tenant on a single-tenant catalog (no picker to choose it through otherwise)
• Registered in NetworkAssembly (TenantStore / TenantProvider)
• TenantStoreTests.swift — 8 unit tests

Not included

• Any real consumer (RequestInterceptor, storage, session lifecycle) — starts in later PRs
• Remote catalog fetch (TenantConfigLoader) — separate PR

Fixed in passing

The original TenantStoreTests.swift was never added to Core.xcodeproj, so it silently never ran as part of CoreTests. Registered it correctly since this PR is what introduces the file.

Testing

Built and ran successfully on device.

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Note

This is the 2nd pull-request of the multi-tenant infrastructure extraction.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (0c72bd4) to head (d1657e7).

Additional details and impacted files

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