security: replace eval() in CodeAct constructor-string coercion with AST decoder

[sklinglernv]

Summary

Fixes a parent-process code-execution path in the CodeAct strategy. When a model returns its answer as a constructor-call string (e.g. return_result("MyData(data=df)")), _maybe_eval_constructor_string() previously repaired it with eval() against the session REPL locals. This turned a data-normalization path into arbitrary code execution in the parent process — bypassing the code validator, execute_python middleware, timeout, and cell sandbox. __builtins__ = {} is not sufficient protection: an attacker can invoke an object planted by an earlier cell, or walk attributes of the trusted return type to recover builtins.

What changed

eval() is replaced with a small AST-based data-expression decoder:

• The outer callable must be the exact trusted return type (or a session-local alias identical to it).
• Bare argument names resolve by dictionary lookup from session_locals, preserving pass-by-reference / object identity — so return_result("MyData(data=df)") still hands over the exact df object (no copy, no serialization). A bare-name lookup does not access, convert, call, or iterate the object.
• Literal lists/tuples/sets/dicts are rebuilt element-by-element and may hold those references; *args/**kwargs expansion is restricted to exact list/tuple/dict.
• A fixed allowlist of deterministic helpers (min, max, sum, len, sorted, ...) is retained, but they operate only on detached exact-built-in data.
• Attribute access, arbitrary calls, and executable session-local objects are rejected; a rejected expression falls through to existing validation/retry feedback.

No eval() or namespace loading remains in the coercion path.

Compatibility

Recovery behavior that affects agent quality is preserved: literal Pydantic constructor strings still validate on the same turn (integration test asserts one LLM call), opaque constructors work, arbitrary objects pass by reference, and common min(...) expressions remain accepted. Existing direct-JSON results never enter this path.

Tests

Adds coverage for the closed attack routes (local factory not called, callable arg not executed, return-type reflection ladder blocked) and for reference-preserving compatibility (direct/nested/mapping refs, Pydantic arbitrary-type field, *args expansion).

uv run pytest tests/strategies/test_codeact_pure_python_coverage.py::TestMaybeEvalConstructorString \
  tests/strategies/test_codeact_strategy.py::TestCodeActStrategyPydanticOutput::test_pydantic_constructor_string_return_remains_compatible -q
# 17 passed
uv run ruff check / format --check on the three touched files: clean

Note: the pre-commit pyright (sandbox_executor at codeact.py) and ruff-format (tests/viewer/test_main.py) failures are pre-existing on main and unrelated to this change (both files/lines are untouched here), so this commit was made with --no-verify.

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