insights: aggregations limit errors for mode availability queries

Created by: chwarwick

Previously if the search query couldn't be parsed checking the availability of the aggregation mode would error. This did not align with what happened when attempting to run the aggregation. That would return a NotAvailable response with details that the search query was not valid. This brings the mode availability in alignment. In cases when it's not determined if the mode is supported false is the default.

It could still be possible for an array of errors to be returned if the query were to ask for the reasonUnavailable and that were to error for multiple modes. I believe this is ok because those errors would be relevant to the specific mode and it is a nullable field.

resolves https://github.com/sourcegraph/sourcegraph/issues/40971

Test plan

Query

{
  searchQueryAggregate(query: "i++ patterntype:regexp",patternType:regexp) {
    modeAvailability {
      mode
      available
      reasonUnavailable
    }
  }
}

Response

{
  "data": {
    "searchQueryAggregate": {     
      "modeAvailability": [
        {
          "__typename": "AggregationModeAvailability",
          "mode": "REPO",
          "available": false,
          "reasonUnavailable": "Grouping is disabled because the search query is not valid."
        },
        {
          "__typename": "AggregationModeAvailability",
          "mode": "PATH",
          "available": false,
          "reasonUnavailable": "Grouping is disabled because the search query is not valid."
        },
        {
          "__typename": "AggregationModeAvailability",
          "mode": "AUTHOR",
          "available": false,
          "reasonUnavailable": "Grouping is disabled because the search query is not valid."
        },
        {
          "__typename": "AggregationModeAvailability",
          "mode": "CAPTURE_GROUP",
          "available": false,
          "reasonUnavailable": "Grouping is disabled because the search query is not valid."
        }
      ],
      "__typename": "SearchQueryAggregate"
    }
  }
}