Tighten the eval/exec/compile dynamic policy so an executing sink (eval/exec/
runpy) applied to a payload that cannot be statically recovered is refused
unconditionally, not only when an RCE-core module happens to be imported in the
snippet. An un-analyzable executing payload can synthesize any shell, network,
or filesystem escape at runtime, so the prior in-scope-import heuristic left
exec(input()) and eval(user_var) allowed whenever no such import was present.
compile() of the same payload stays allowed since it does not run.
A payload that is fully recovered as a constant but is invalid Python for the
sink's mode (for example eval("data = 1") or eval("not python !!")) is now
allowed: its exact source is known and it raises SyntaxError at runtime, so it
is not an execution vector. Only genuinely opaque, non-recoverable payloads
(for example eval of a runtime-computed f-string) reach the block.
Remove the now-unused _RCE_CORE_MODULES set and _scope_imported_roots helper,
and move the opaque-f-string case in the tests to the blocked set.
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