Say why an action field was ignored

[thc1006]

From a review of #117: the catch around the action conversion is except Exception, so a defect in our own code, or a third-party exception, is reported
as ordinary invalid input.

Three parts to that, and they are not equally true.

The catch should stay broad. It is there because the list cannot be guessed.
A torch tensor still carrying its graph raises RuntimeError on np.asarray, an
action that is not a mapping raises TypeError on the lookup, and every narrowing
so far has been one of those found the hard way. except Exception also does not
catch KeyboardInterrupt or SystemExit, which are BaseException.

But the message was the problem, and this fixes it. The log said:

Step 42: ignoring tvc, which the environment cannot use (1 such steps for that field)

Nothing about why. A competitor sending "forward" and a NameError in the four
lines above produced that same sentence, so the defect was invisible. Now:

Step 42: ignoring tvc, which the environment cannot use:
ValueError: could not convert string to float: 'forward' (1 such steps for that field)

_usable_action_fields returns a mapping of field to reason instead of a set of
names. check_action still answers with a sorted list of names, because that is
the public thing an agent calls:

if check_action(action):
    action = last_good_action

The __array__ part does not apply here. np.asarray does call __array__ on
whatever it is handed, so an expensive or side-effecting one runs. That object comes
from the competitor's own agent, in the competitor's own process, and get_action
can already run anything. Nothing here re-executes a submitted agent: I checked, and
agent_module_file is carried in the submission but never imported or executed. If
that ever changes, the sandbox question is much bigger than this function.

Tests

Eight, in tests/test_unusable_action_reasons.py. Drop the reasons and go back to a
set: six fail, and the two check_action tests pass, which is the point of having
them.

a raised exception keeps its type	fails
an unexpected exception type survives	fails
the wrong count says what was expected	fails
a non-finite value says so	fails
a missing field keeps its lookup error	fails
the reason reaches the log, through env.step	fails
check_action is a sorted list of names	passes
a usable action is an empty list	passes

Local CI is green: ruff check, ruff format, uv lock --check, 506 passed and 22
skipped.

[thc1006]

@zuorenchen this sits on the same ground as #117 which you said went a bit too deep, so please close it if you would rather not carry it; all it does is make the existing log line name the conversion that failed, so a competitor can see what to fix in their own agent.

[zuorenchen]

LGTM, let's merge it as it is working nicely