Describe the bug

A magic trailing comma correctly explodes a PEP 695 type-parameter list of plain type vars (one per line), but is silently ignored once the list contains a *TypeVarTuple or **ParamSpec — the params collapse back onto a single line.

To Reproduce

class C[
    T,
    *Ts,
    **P,
]:
    pass

Running black --target-version py312 (stable style) produces:

class C[
    T, *Ts, **P,
]:
    pass

The same construct with only plain type vars is exploded as expected:

class C[
    T,
    U,
    V,
]:
    pass

It reproduces on def/async def/type aliases too, and regardless of --target-version (autodetected). In a def, the argument list still explodes correctly while the type-param list collapses, e.g. def f[T, *Ts, **P,](x): -> the [...] collapses but (x,) explodes.

Expected behavior

The magic trailing comma should force the type-parameter list to explode one-per-line, identical to the plain-type-var case, whether or not the list contains a *TypeVarTuple/**ParamSpec.

Environment

• Black's version: 26.5.2.dev29+gb74b23013 (commit b74b230), also reproduces on current main
• OS and Python version: Linux, CPython 3.14

Additional context

The collapsed output is stable and AST-equivalent, so it does not trip idempotency/AST-safety checks — it's purely a splitting-decision bug specific to starred type params.