test(pins): widen whitelist for the episode-collections JSON shapes (item 2 series)

TestContractPin_CollectionsJSON tripped on the additive keys the
episode-collections work introduced — the tripwire worked exactly as
designed (additive keys must be whitelisted here AND in
docs/contract_pins.md first).

- Collections envelope gains rule_match_mode (item 2.4): the
  smart-collection match semantics on GET /api/collections/:id.
- BookInfo gains four STABLE keys (item 2.3): is_episode,
  feed_title, download_state, hidden — dropped omitempty so every
  card surface emits the same key set (the viewer_percentage
  precedent; adapters that predate episode awareness emit zero
  values).

docs/contract_pins.md changelog entry written. Nothing removed,
nothing renamed.

cmd/server/tests/contract_pins_test.go

@@ -459,6 +459,8 @@ var bookInfoKeys = []string{
 	"media_item_id", "title", "author", "cover_image_path",
 	"has_conflict", "format_group", "badge",
 	"viewer_percentage", "library_name",
+	// Episode-collections extras (2.3 widening, every surface emits):
+	"is_episode", "feed_title", "download_state", "hidden",
 }
 
 func collectionURL(setup *TestServerSetup, id string) string {
@@ -500,6 +502,7 @@ func TestContractPin_CollectionsJSON(t *testing.T) {
 		"id", "user_id", "name", "description", "color", "icon",
 		"auto_assign_rules", "view_settings", "books", "book_count",
 		"is_smart", "created_at", "up_next",
+		"rule_match_mode", // item-2.4 widening (docs/contract_pins.md)
 	}
 
 	t.Run("user collection with library_id (badged branch)", func(t *testing.T) {