Preserve model cache on empty server responses

[Copilot]

The model metadata fetcher cleared its cache when the server returned an empty but successful response, leaving users without available models during transient failures.

Changes

• Guard against empty responses: Skip cache updates when data.length === 0 and existing models are present
• Reset fetch timer: Set _lastFetchTime = 0 on empty responses to enable immediate retry instead of waiting 10 minutes
• Log warning: Added diagnostic logging when preserving cache due to empty response

Implementation

const data: IModelAPIResponse[] = (await response.json()).data;

// Preserve last-known-good cache if server returns empty list
if (data.length === 0 && this._familyMap.size > 0) {
	this._logService.warn(`Server returned empty model list, preserving existing cache ${requestId}`);
	this._lastFetchTime = 0;  // Allow immediate retry
	return;
}

// Build and swap in new maps...

This follows the existing pattern for rate-limit handling at line 260.

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