kleidiai : fix MUL_MAT support for batched (3D) inputs (llama/20620)

kleidiai : fix MUL_MAT support for batched (3D) inputs (llama/20620)

* kleidiai : fix MUL_MAT support for batched (3D) inputs

The supports_op() check incorrectly rejected MUL_MAT operations with 3D
inputs (ne[2] > 1), but the actual compute_forward_qx() implementation
handles batched inputs correctly via a loop over ne12.

This caused models with Q4_0/Q8_0 weights to crash during graph scheduling
when n_seq_max > 1, because weights were placed in KLEIDIAI buffers during
loading (tested with 2D inputs) but the runtime used 3D inputs.

Also relax the buffer check to allow supports_op() to be called during
weight loading when src[0]->buffer is NULL.

Fixes #20608

* Kleidiai support_ops should only return true for 3D inputs, not also 4D