Fix TryGetLocationForGatewayDiagnostics to use server-side multi-master setting

[Copilot]

TryGetLocationForGatewayDiagnostics was gated on CanUseMultipleWriteLocations(), which requires both the client opt-in (UseMultipleWriteLocations) and the account setting (EnableMultipleWriteLocations). This caused the method to return null for the region name when using a multi-master account with the client opt-in disabled — a valid configuration that should still produce correct diagnostics.

Changes

• LocationCache.cs: Replace CanUseMultipleWriteLocations() with this.enableMultipleWriteLocations in TryGetLocationForGatewayDiagnostics. The hub write region fallback for the default endpoint is an account-topology concern, not a client routing policy concern.

• LocationCacheTests.cs: Add ValidateTryGetLocationForGatewayDiagnosticsOnDefaultEndpointForMultiMasterWithClientOptOut to cover the previously untested asymmetric case: account has EnableMultipleWriteLocations = true but client has UseMultipleWriteLocations = false.

// Before: returned null regionName when client opt-in was disabled,
// even though the account was multi-master
if (this.CanUseMultipleWriteLocations())  // requires client opt-in AND account setting

// After: keyed on account/server setting only
if (this.enableMultipleWriteLocations)

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[NaluTripician]

Hi @NaluTripician — this PR has been idle for ~62 days. (Copilot-authored sub-PR of #5618, which you own) What's the current ETA / status? Flagging as part of an open-PR cleanup pass; if it's no longer being pursued we can close it. No auto-close — just looking for a status update.