perf(clickhouse): stop storing native event attributes (#4866)

## Summary

Stop storing native JSON event attributes while retaining the existing
materialized `attributes_text` representation. Also remove the two
unused full-text indexes from the source event table.

## Design

This depends on
[#4860](#4860). Its
explicit-column writer must be deployed before this migration is applied
so attributes continue to feed the materialized string.

The JSON input becomes EPHEMERAL, eliminating its stored subcolumns and
merge overhead for new parts. Existing parts retain their JSON files
until normal merges or TTL removal.

The migration is intentionally not reversible automatically. Restoring
native JSON storage safely requires inspecting the current schema and
coordinating the writer change, so it should be done with a new forward
migration rather than a retry-sensitive Down migration.

Author: carderne

internal-packages/clickhouse/schema/042_reduce_task_events_v2_storage_overhead.sql

@@ -0,0 +1,21 @@
+-- +goose Up
+
+-- Full-text search is served outside the source event table. Keeping these
+-- indexes here adds work to every event insert and merge without serving reads.
+ALTER TABLE trigger_dev.task_events_v2
+    DROP INDEX IF EXISTS idx_attributes_text_search;
+
+ALTER TABLE trigger_dev.task_events_v2
+    DROP INDEX IF EXISTS idx_message_text_search;
+
+-- attributes remains an insert input for attributes_text, but is no longer
+-- stored. Writers must include attributes in an explicit insert column list
+-- because implicit INSERT column lists exclude EPHEMERAL columns.
+ALTER TABLE trigger_dev.task_events_v2
+    MODIFY COLUMN attributes JSON EPHEMERAL;
+
+-- +goose Down
+
+-- Restoring the stored JSON column safely requires inspecting the live schema
+-- and coordinating the writer rollback. Use a new forward migration instead.
+SELECT throwIf(1, 'This migration cannot be rolled back automatically');

internal-packages/clickhouse/src/taskEvents.test.ts

@@ -62,6 +62,49 @@ describe("task events v2", () => {
           has_inserted_at: 1,
         },
       ]);
+
+      const readColumnKinds = ch.reader.query({
+        name: "read-task-event-attribute-column-kinds",
+        query: `SELECT name, default_kind, default_expression
+        FROM system.columns
+        WHERE database = 'trigger_dev'
+          AND table = 'task_events_v2'
+          AND name IN ('attributes', 'attributes_text')
+        ORDER BY name`,
+        schema: z.object({
+          name: z.string(),
+          default_kind: z.string(),
+          default_expression: z.string(),
+        }),
+      });
+      const [columnError, columns] = await readColumnKinds({});
+      expect(columnError).toBeNull();
+      expect(columns).toEqual([
+        {
+          name: "attributes",
+          default_kind: "EPHEMERAL",
+          default_expression: "defaultValueOfTypeName('JSON')",
+        },
+        {
+          name: "attributes_text",
+          default_kind: "MATERIALIZED",
+          default_expression: "toJSONString(attributes)",
+        },
+      ]);
+
+      const readRemovedIndexes = ch.reader.query({
+        name: "read-removed-task-event-text-indexes",
+        query: `SELECT name
+        FROM system.data_skipping_indices
+        WHERE database = 'trigger_dev'
+          AND table = 'task_events_v2'
+          AND name IN ('idx_attributes_text_search', 'idx_message_text_search')
+        ORDER BY name`,
+        schema: z.object({ name: z.string() }),
+      });
+      const [indexError, indexes] = await readRemovedIndexes({});
+      expect(indexError).toBeNull();
+      expect(indexes).toEqual([]);
     }
   );
 });