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>

tests/testthat/test-recipes.R

@@ -392,6 +392,26 @@ test_that("I1 + #34: cog_revenue never suggests expenditure-only recipes", {
   expect_false("corrections_combined" %in% ids)
 })
 
+test_that(".query_candidate_recipes() scopes candidates by category_type (#34)", {
+  # Direct assertion on the mechanism the two tests above exercise
+  # end-to-end: corrections_combined's own components (E04/E05) are
+  # category_type = 'expenditure' in summary_categories, so an
+  # expenditure-flavored flow_prefixes call must surface it and a
+  # revenue-flavored one must not. This queries only summary_categories/
+  # harmonization_recipes (no government data), so it runs against the
+  # bundled fixture with no skip_if_no_corpus() needed.
+  con <- uscogdata:::.ensure_session()
+
+  expenditure <- uscogdata:::.query_candidate_recipes(
+    con, category = "Corrections", flow_prefixes = c("E", "F", "G"))
+  expect_true("corrections_combined" %in% expenditure)
+
+  revenue <- uscogdata:::.query_candidate_recipes(
+    con, category = "Corrections",
+    flow_prefixes = c("T", "A", "U", "B", "C", "D"))
+  expect_false("corrections_combined" %in% revenue)
+})
+
 test_that("uscogdata#9: no partial-coverage fire in a modern year", {
   skip_if_no_corpus()
   r <- cog_spending("061037123085", years = 2019L, category = "Public Welfare")