test: collect the unregistered fast CPU tests and update cpu ci python to 3.12

[Rockdu]

What

• Drop the monkeypatch.setattr(backend, "_is_primary", False) line from test_collector_startup_failure_disables_dashboard.
• Register the three tests/fast files that no CI registration marker covered, so the suite runner collects them.

Why

miles/dashboard/backend.py has never had an _is_primary attribute — it exposes _handle and _resolution_failed, both since the module landed in #58. monkeypatch.setattr defaults to raising=True, so that line raises AttributeError and the test has been failing since the day it was merged. The meaningful part of the setup is the _handle = None patch right above it, so deleting the stale line restores the intended assertion (actor creation raises -> init_dashboard returns False and warns).

It went unnoticed because tests/ci/run_suite.py globs tests/fast/**/test_*.py but collect_tests only keeps files whose source contains a register_*_ci(...) marker (parsed via AST). Three files had none, so CI never ran them:

File	Was collected?
tests/fast/dashboard/test_async_collector.py	no — and it was red
tests/fast/utils/test_diffusion_rollout_response.py	no — green
tests/fast/utils/test_processing_utils.py	no — green

Registering them means a future stale patch like this one fails a PR instead of only failing for whoever happens to run pytest tests/fast by hand.

Registering test_async_collector.py also exposed that miles/dashboard/events.py and store.py use enum.StrEnum (3.11+) while stage-a-cpu — the one job that does not run inside the container image — set up Python 3.10 to match setup.py's python_requires=">=3.10". The declaration and the code have disagreed since the dashboard landed. The dev/CI container image is Python 3.12, so the third commit raises the declared floor to 3.12 (setup.py, classifiers, isort py_version, and the two workflow jobs that pin a bare-runner Python) rather than adding a dependency or shimming StrEnum. It is the same commit as in #84, which needs it for the test it adds; whichever lands first makes it a no-op in the other.

Validation

• collect_tests now returns all three as HWBackend.CPU / stage-a-cpu / est_time=15 / labels=[].
• pytest tests/fast -q -> 138 passed (was 137 passed + 1 failed).
• pre-commit run --all-files passes; isort accepts the marker-before-imports layout already used by tests/fast/utils/test_misc.py.

Commits are ordered fix-then-register so the registration never lands while the test is red.

Files

• tests/fast/dashboard/test_async_collector.py — stale monkeypatch removed, CPU CI registration added.
• tests/fast/utils/test_diffusion_rollout_response.py — CPU CI registration added.
• tests/fast/utils/test_processing_utils.py — CPU CI registration added.
• setup.py, pyproject.toml, .github/workflows/_run-ci.yml, .github/workflows/pre-commit.yml — python floor raised to 3.12.

Checklist

• pre-commit run --all-files passes
• Added/updated tests for new behaviour — this PR is the test-side change
• pytest -x is green — pytest tests/fast is 138 passed
• If launch flags changed, python3 train.py --help still parses — n/a, no flag changes
• If a public flag was added, it appears in the CLI reference docs — n/a, no new flag
• If an example was added, it has a real walkthrough — n/a, no new example