ggml-quants : avoid invalid rounding in qkx3 scale search (#29817)

* ggml-quants : avoid invalid rounding in qkx3 scale search

The imatrix scale search can produce an infinite, NaN, or otherwise out-of-range value when the fitted minimum collapses to the maximum or makes the range extremely small. That value is then passed to nearest_int and can trip its assertion in Debug builds.

Clamp the quantization level to [0, nmax] before rounding so valid in-range values behave the same as before while invalid scale-search results no longer reach nearest_int.

Add regression coverage for degenerate imatrix groups across q2_K, q4_K, q5_K, q4_1, and q5_1.

Fixes #29804.

Assisted-by: Claude Opus 5.5

* tests: print degenerate imatrix quant types