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Stepper: stop sending a fabricated REPL result after a run (#422)

py-slang#421: running a program through the Stepper tab left a stale
string (e.g. "3" for `1 + 2`) in the host's REPL even after switching
to a different evaluator without re-running anything. Python programs
run in exec mode -- a script never produces a value the way a REPL
expression does, and every other evaluator here (Py2JsEvaluator,
PyodideEvaluator) already reports sendResult(undefined) for exactly
that reason.

PyStepperEvaluator was the one exception: runChunk separately
re-derived and sent the text of the program's *last expression
statement* via a second evaluatePython pass, mirroring js-slang's
SourceStepperEvaluator -- appropriate there, since Source is
expression-oriented and a program's last expression genuinely is "the
result", but wrong for Python's exec model. The Stepper tab's own step
sequence already shows the program's final state; there was nothing
this second pass needed to report to the REPL in the first place.

runChunk now sends undefined, matching every other evaluator, and
never re-runs the program a second time. This also removes the
InputRecorder machinery entirely: it existed solely to keep an
input()'s answer consistent between sendSteps and the now-deleted
second evaluatePython pass, so with only one pass left, requestInput
wires directly to the real host round-trip.

Also adds PyStepperEvaluator.test.ts, which didn't exist before --
confirmed each new test genuinely reproduces the reported leak with
the fix reverted.


Claude-Session: https://claude.ai/code/session_019GKx36me2L9FrkDhqvHJ6X

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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