test: stabilize external collection cancel test (#50074)

#### What this PR does / why we need it:

Stabilizes `TestExternalCollectionManager_CancelTask` by waiting for the
asynchronous manager state update after cancel is observed by the task
function.

`CancelTask` only triggers context cancellation. The task goroutine
records `JobStateFailed` after `taskFunc` returns with
`context.Canceled`, so reading the task info immediately after observing
`ctx.Done()` can race with `UpdateResult` and intermittently see
`JobStateInProgress` with an empty failure reason.

This test now waits until the manager snapshot reports `JobStateFailed`,
then asserts the failure reason separately for clearer failure output.

#### Which issue(s) this PR fixes:

N/A

#### Special notes for your reviewer:

This is a test-only flaky fix.

#### Tests

- `gofumpt -l internal/datanode/external/manager_test.go`
- `git diff --check HEAD~1..HEAD`
- `go test -v -count=1 -tags dynamic,test -gcflags="all=-N -l"
-ldflags="-r ${MILVUS_WORK_DIR}/cmake_build/lib -r
${MILVUS_WORK_DIR}/internal/core/output/lib"
./internal/datanode/external -run
'TestExternalCollectionManager_CancelTask' -timeout 300s`
- `go test -v -count=1 -tags dynamic,test -gcflags="all=-N -l"
-ldflags="-r ${MILVUS_WORK_DIR}/cmake_build/lib -r
${MILVUS_WORK_DIR}/internal/core/output/lib"
./internal/datanode/external -timeout 300s`

Signed-off-by: sijie-ni-0214 <sijie.ni@zilliz.com>