fix(suggestions): guard empty candidates and restore year typing in .query_covered_years()

Addresses roborev jobs 205/208 (reviewing pre-merge draft/original commits
of #33/#34, now split and merged as #69/#70):

- Restore `res$year <- as.integer(res$year)` in .query_covered_years(),
  dropped during the #33 rebuild as apparently-dead code. Its own roxygen
  promises an integer `year` column; without the coercion the
  implementation no longer matches that documented contract.
- Guard .query_covered_years() against empty `candidates`, mirroring the
  existing `gap_years` guard. Not reachable via .build_suggestions() today
  (candidates is checked non-empty before this is called), but the
  extracted helper is independently callable and previously built a
  malformed `WHERE r.recipe_id IN ()` clause for a hypothetical direct
  caller with no candidates.
- Add a direct fixture-only unit test of .query_candidate_recipes()'s
  category_type filtering (#34) in both flow directions -- queries only
  summary_categories/harmonization_recipes metadata, so it runs without
  skip_if_no_corpus(), unlike the two existing end-to-end tests.

1080 tests pass (2 skipped live-corpus).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

R/suggestions.R

@@ -300,14 +300,14 @@
 #'   result.
 #' @return Data frame with columns `recipe_id` (character) and `year`
 #'   (integer). Returns an empty data frame (`recipe_id = character(0)`,
-#'   `year = integer(0)`) when `gap_years` is empty, so callers can safely
-#'   reference `$recipe_id`.
+#'   `year = integer(0)`) when `gap_years` or `candidates` is empty, so
+#'   callers can safely reference `$recipe_id`.
 #' @noRd
 .query_covered_years <- function(con, candidates, cohort, gap_years) {
-  if (length(gap_years) == 0L) {
+  if (length(gap_years) == 0L || length(candidates) == 0L) {
     return(data.frame(recipe_id = character(0), year = integer(0)))
   }
-  DBI::dbGetQuery(con, sprintf(
+  res <- DBI::dbGetQuery(con, sprintf(
     "SELECT DISTINCT r.recipe_id, l.year
      FROM long l
      JOIN harmonization_recipes r
@@ -322,6 +322,8 @@
     .sql_lit_chr(candidates), .cohort_sql(cohort, "l.canonical_govid"),
     paste(gap_years, collapse = ",")
   ))
+  res$year <- as.integer(res$year)
+  res
 }
 
 #' Query recipe metadata: labels and year spans for a set of candidate