fix(ui): stop handler tests from booting real Canton stacks

[zheli]

Follow-up to #322, which sealed only the start path.

Problem

handleCreate, handleResumeInstance and realRecreateWork called localnet.RunUp directly on a detached context.Background() goroutine. Any test reaching them booted a real Canton stack that outlived the test run.

Confirmed empirically: on a clean host, make test on main leaks 9 containers under compose project canton-pebble, traced via container labels to the devkit-handlers-tests-* tempdir.

Those stranded containers then make later runs take the fast-start branch and return 204/500 instead of 202. That is the failure #322 diagnosed — it looked "pre-existing" on a developer machine while CI stayed green, because CI runners start clean.

Fix

• Route the three remaining RunUp call sites through the package-level seam fix(ui): keep the start-handler test from booting a real Canton stack #322 already added.
• Default runUp to a no-op in TestMain, so hermeticity is structural — a test added later cannot reintroduce the leak. This mirrors the existing runPreflightForVersion stub, which is process-wide for the same reason.
• Move the seam var block above handleStartInstance so its godoc is no longer separated from the func (wart from fix(ui): keep the start-handler test from booting a real Canton stack #322).

Two latent flakes surfaced

TestCancelUp_HappyPath and TestCreate_DuplicateNameReturns409 silently depended on the real RunUp being slow (pulling images) to keep a job in-flight. Both now install a blocking stub that returns on ctx.Done(), matching their existing jobs.Cancel + waitJobsDrain teardown. TestCreate_DuplicateNameReturns409 was failing under -race on main for this reason.

TestResume_StoppedInstance202 now asserts what its own comment said it could not ("we cannot easily assert the goroutine reaches RunUp"): that bring-up receives the recorded version, pinning the no-silent-upgrade rule.

Verification

• make test exit 0, container delta 0 (was +9 on main)
• go test -race -count=3 green; plain -count=3 green
• make lint -> 0 issues.

No user-facing behaviour change, so no docs/changes-from-proposal.md entry (internal indirection + tests only).