ci: add ASan coverage for hook-built native assets

The upstream work needed to run hook-built native assets under sanitizer runtimes has landed:

- dart-lang/sdk#63690 added shared Linux CLI runtimes.
- dart-lang/test#2708 added `dart test -c cli` support for ASan, MSan, and TSan.
- dart-lang/sdk#63987 added end-to-end SDK coverage using a hook-built native asset.

The remaining release dependency is a published `package:test` version containing dart-lang/test#2708. The change is currently on `package:test` master under `1.32.0-wip`, while the latest published version still predates it.

Once that version is available:

- add a Linux ASan CI lane using `dart test -c cli -p vm-asan`;
- exercise the existing FFI memory-pressure workload through the public API;
- share the workload with the existing Valgrind target rather than duplicating it;
- keep the Valgrind lane, since it provides complementary coverage;
- force native assets to build from source if packaged prebuilts are enabled, so the native code is sanitizer-instrumented.

ASan should be the first lane. MSan and TSan can follow once the ASan job is stable and their runtime behavior has been evaluated.

Start by locating the existing Valgrind target and FFI memory-pressure workload, then inspect the CI configuration and the published package:test dependency. Confirm the required package:test version is available before evaluating the ASan command and native-asset build behavior. Done means an ASan Linux lane exercises the shared workload while the Valgrind lane remains.

Written by the indexing model from the issue text.