Avoided needless Semantic Search calls when no data source can be searched (#1108)

documentation/Tools.md

@@ -212,7 +212,7 @@ The tool offers the data sources of the chat which the provider may use. With th
 
 An ERI data source has a single page, see `IDataSource.GetLastPage`, and asks its server for exactly as many matches as the classic process does. ERI v1 knows no offset, so a further page would have to ask for all pages before it as well, and a server which allows only the configured number of matches would refuse it: the model would hear that the data source could not be searched rather than that there is nothing more.
 
-An ERI data source whose server does not answer the check of `DataSourceService` (sign-in or security requirements failing, or slower than six seconds) cannot be offered, since its server's rules are unknown. It is not left out silently either: `AllowedSelectedDataSources.UnreachableDataSources` lists it when its configured security policy allows it for the provider, and the description of the tool names it, without an ID, as not reachable or not signed in to right now. The tool is offered even when every data source of the chat is unreachable, and a search of all data sources reports those as not searched, again without an ID. Otherwise, the model would answer from its own knowledge as though the chat had no data sources. The classic process names the same data sources with the same words after the passages it hands the model, also when it found nothing: `AllowedSelectedDataSources.GetUnreachableOfTheChat` decides which ones count for both, and `UnreachableDataSources` writes the note.
+An ERI data source whose server does not answer the check of `DataSourceService` (sign-in or security requirements failing, or slower than six seconds) cannot be offered, since its server's rules are unknown. It is not left out silently either: `AllowedSelectedDataSources.UnreachableDataSources` lists it when its configured security policy allows it for the provider, and the description of the tool names it, without an ID, as not reachable or not signed in to right now. A search of all data sources reports those as not searched, again without an ID. Otherwise, the model would answer from its own knowledge as though the chat had no data sources. When no data source of the chat can be searched at all, the tool stays out of the request: the model would call it all the same, and the call would cost a round of the answer without searching anything. The classic process, which steps back for Semantic Search, then writes the note into the system prompt instead; it asks `SemanticSearchTool.GetOfferedDataSourcesAsync`, so that the note appears exactly when the tool stays out. When it searches itself, the classic process names the same data sources with the same words after the passages it hands the model, also when it found nothing: `AllowedSelectedDataSources.GetUnreachableOfTheChat` decides which ones count for both, and `UnreachableDataSources` writes the note.
 
 Every check of an ERI data source signs in to its server, and one message checks several times: when the tool is offered, before every search, and with every message of the classic process. `DataSourceService` therefore keeps a server's answer for one minute in an `ERISecurityCheckCache` and joins a check which is still running, so that a server which limits sign-ins does not start refusing. A server which did not answer is asked again after 15 seconds, and a changed configuration of the data source asks anew right away. Expired answers are dropped whenever a check starts. The minute holds for an answer which lets the data reach any provider as well, on purpose: when an operator opened a server by mistake and corrects it, every AI Studio follows within that minute, and until then, a chat with a cloud provider may still search the server. The cache lives only as long as the app runs, so no answer sticks for longer.