ci: disable the testcontainers reaper in go-unit-tests

[jjamroga]

Problem

go-unit-tests intermittently fails with two Postgres-backed packages timing out before any container starts:

migrate_test.go:255:         failed to start postgres container: ... reaper: from container "1c88b27d": wait for reaper 1c88b27d: context deadline exceeded
migration_000007_test.go:32: startTestDB: start container: ...     reaper: from container "1c88b27d": wait for reaper 1c88b27d: context deadline exceeded

FAIL github.com/kagent-dev/kagent/go/core/pkg/cli/db/migrate  61.298s
FAIL github.com/kagent-dev/kagent/go/core/pkg/migrations       114.543s

Neither is an assertion failure — both are 60s timeouts waiting on the testcontainers Ryuk reaper, so no Postgres container is ever created and the tests fail without evaluating anything.

Cause

go test ./... runs each package as its own process, so every process using internal/dbtest contends for one session-scoped Ryuk reaper. The logs show the race directly — earlier packages obtain that exact container, then later ones wait 60s for the same ID and give up:

19:14:39 ⏳ Waiting for Reaper "1c88b27d" to be ready
19:14:40 🔥 Reaper obtained from Docker for this test session 1c88b27d
19:15:00 ⏳ Waiting for Reaper "1c88b27d" to be ready      <- times out

Observed on unrelated PRs and on main:

• run 32763005552 (PR feat(app): let an extension supply the A2A handler #2544, reaper 1c88b27d)
• run 32639919261 (main, reaper 69c2d478) — same two tests, same error

Fix

Set TESTCONTAINERS_RYUK_DISABLED=true on the step.

Ryuk exists to reap containers leaked by a test run that dies without cleaning up. GitHub runners are ephemeral and discarded after the job, so that cleanup has no value here, and dbtest already terminates its containers via t.Cleanup. Disabling the reaper removes the contended singleton without giving up anything CI relies on.

Scoped to go-unit-tests: it is the only job running testcontainers-backed tests. The e2e job runs just core/test/e2e against a Kind cluster.

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