fix: Crash when hovering on anonymous consts

Previously, the following code would cause rust-analyzer to infinitely
recurse when hovering on `x`.

    trait Tr<const N: usize> {}
    pub fn f(x: impl Tr<{ 0 }>) {}

Attaching gdb and printing a stack trace shows that the problem is in
HirDisplay for an anonymous const.

    frame 9  hir_ty::display::hir_fmt_generic_arguments () at crates/hir-ty/src/display.rs:1920
    frame 10 0x00005555563b9f94 in hir_ty::display::hir_fmt_generics () at crates/hir-ty/src/display.rs:1843
    frame 11 0x00005555563bd0ea in <hir_ty::next_solver::consts::Const as hir_ty::display::HirDisplay>::hir_fmt () at crates/hir-ty/src/display.rs:768
    frame 12 0x00005555563ba964 in <hir_ty::next_solver::generic_arg::GenericArg as hir_ty::display::HirDisplay>::hir_fmt () at crates/hir-ty/src/display.rs:716
    frame 13 hir_ty::display::hir_fmt_generic_arguments () at crates/hir-ty/src/display.rs:1920

Instead, just render anonymous consts as `{const}` rather than trying
to render the synthetic type parameters too.

(We could track cycles like we do for recursive bounds, but that just
produces a hover of `x: impl Tr<{const}<impl …>> + ?Sized` which
doesn't seem terribly useful.)

AI disclosure: Initial implementation done with Codex and GPT-5.5.