fix: return an error instead of panicking when the frame stack empties

[jtippett]

Summary

A guest input that empties the VM frame stack panics execute() (frames.last().unwrap()), aborting the whole host process. For an interpreter whose purpose is running untrusted, agent-generated code, guest input should never be able to take down the host. The VM now returns a RuntimeError to the caller instead.

Two minimal reproductions (both abort the process on current master):

// a throw escaping a nested array callback
try { [1].map(a => [2].map(b => { throw "n"; })); } catch (e) {}

// a throw escaping a callback inside a class method
class A { run() { return [1].map(x => { throw "m"; }); } }
try { new A().run(); } catch (e) {}

Note: with this fix the run surfaces RuntimeError("no active call frame") rather than the guest catch observing the throw — the frame-unwinding accounting for callback frames looks like a separate, pre-existing issue (happy to file it with details). This PR only removes the process abort.

Changes

• The three frames.last().unwrap() dispatch sites return a RuntimeError on an empty frame stack instead of panicking.
• Two regression tests in tests/error_handling.rs, verified to abort the process without the fix.

Test plan

• Unit tests pass (cargo test)
• make lint clean (fmt + clippy -D warnings)
• New regression tests fail (abort) on master, pass with the fix
• TC39 Test262 sweep (~48k inputs): zero panics after this change; previously aborted on the first such input
• CI passes

Related issues

None open — found while running Test262 against the VM. We run this patch in production via ex_zapcode (Elixir bindings pinned to a zapcode-core fork).

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📥 Commits

Reviewing files that changed from the base of the PR and between ebc06fd and 28edb17.

📒 Files selected for processing (2)

• crates/zapcode-core/src/vm/mod.rs
• crates/zapcode-core/tests/error_handling.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[TheUncharted]

Hello @jtippett
Thanks for the PRs you created I will look this week end :)