bug: Starlark result conversion is lossy — big ints become 0, tuples/sets/bytes become nil

[robbyt]

Summary

ConvertStarlarkValueToInterface in engines/starlark/internal/converters.go silently loses data for several legal result types. Scripts return valid values; Interface() returns a wrong value or nil and only logs an error.

1. Big ints convert to 0

engines/starlark/internal/converters.go:59-61:

case starlarkLib.Int:
	i, _ := v.Int64()   // ok flag discarded
	return i, nil

starlark-go's Int64() returns (0, false) for ints that don't fit in int64. Discarding ok yields 0.

Repro (confirmed): _ = 1 << 64 → Inspect() reports 18446744073709551616, Interface() returns 0.

Fix: check ok; on failure fall back to v.BigInt() (*big.Int), a float, a string, or an explicit error — not a silent 0.

2. Tuple, set, and bytes results become nil

ConvertStarlarkValueToInterface (converters.go:53-102) has no case for starlark.Tuple, *starlark.Set, or starlark.Bytes, so they hit the default branch and return an error; Interface() (engines/starlark/evaluator/response.go:101-108) logs it and returns nil. Meanwhile Type() (response.go:50-77) advertises "tuple" → data.TUPLE and "set" → data.SET, so Type() and Interface() disagree.

Repro (confirmed):

• _ = (1, 2, 3) → Type() == data.TUPLE, Interface() == nil
• _ = set([1, 2]) → Type() == data.SET, Interface() == nil
• _ = b"abc" → Interface() == nil, and Type() == data.ERROR ("bytes" is also unmapped in the Type() switch, hitting its default at response.go:73-75)

Fix: add conversion cases — starlark.Tuple → []any, *starlark.Set → []any, starlark.Bytes → []byte or string. For the Type() switch, note there is no data.BYTES constant (platform/data/types.go), so either map "bytes" → data.STRING or add a BYTES constant.

Impact

High — legal script results are silently corrupted (big ints) or dropped to nil (tuple/set/bytes) while the type system reports success. No existing test covers these cases.

Fix

Handle all four types in ConvertStarlarkValueToInterface, resolve "bytes" in Type(), and add table tests for big-int, tuple, set, and bytes results asserting Type() and Interface() agree.