simics-macro: incorrect idx_type for Vec/BTreeSet attributes causes SIMICS warning

## Summary

When registering SIMICS class attributes with `Vec` or `BTreeSet` types via the `#[class(attribute(...))]` proc macro, SIMICS emits the following warning at load time:

```
*** Warning: attribute in class not indexed, ignoring non NULL indexed value type
```

## Root Cause

In `simics-macro/src/class/mod.rs` ([line 333](https://github.com/intel/simulator-bindings/blob/main/modules/simics-macro/src/class/mod.rs)), the `ty_to_typestring()` function unconditionally returns `Some(TypeStringType::Integer)` as the `idx_type` for `Vec` and `BTreeSet` types:

```rust
"Vec" | "BTreeSet" => {
// ...
(
quote!(simics::TypeStringType::List(
vec![simics::TypeStringListType::ZeroOrMore(Box::new(#inner))]
)),
Some(quote!(simics::TypeStringType::Integer)), // <-- should be None
)
}
```

This `idx_type` is passed through to `SIM_register_typed_attribute`, telling SIMICS that the attribute supports indexed access with integer keys. However, the attribute is not actually registered with indexed getter/setter callbacks — it is a plain list attribute. SIMICS detects the mismatch and emits the warning.

## Suggested Fix

Return `None` instead of `Some(...)` for the `idx_type`:

```rust
"Vec" | "BTreeSet" => {
// ...
(
quote!(simics::TypeStringType::List(
vec![simics::TypeStringListType::ZeroOrMore(Box::new(#inner))]
)),
None,
)
}
```

## Impact

The warning is cosmetic — SIMICS handles it gracefully and the attribute works correctly as a plain list. However, it is confusing for users and produces noisy output on module load.

## Reproduction

Any SIMICS module using `#[class(attribute(...))]` with a `Vec` or `BTreeSet` field will trigger this warning. For example:

```rust
#[class(attribute(optional))]
pub exceptions: BTreeSet,
```

Discovered in the [tsffs](https://github.com/intel/tsffs) project.