[operators, subgraph] Harden operator reshape workspace initialization and subgraph tensor rank/axis validation

[anagnorisis2peripeteia]

Summary

This PR addresses several edge-case defects across operator reshape routines and subgraph runtime reshaping:

1. Operator Reshape Workspace Sizing & Safety:

   • fully-connected-nc, batch-matrix-multiply-nc: Initialize *workspace_size = 0 unconditionally upon entry when workspace_size != NULL.
   • dynamic-fully-connected-nc: Set *workspace_size = 0 when batch_size == 0, and check if (workspace_size != NULL) prior to computing workspace requirements.
   • resize-bilinear-nhwc: Set *workspace_size = 0 when batch_size == 0.
   • pack-lh: Set *output_size_bytes = 0 when num_groups == 0 || batch_size == 0.
   • reduce-nd: Set *workspace_size = 0 when num_output_elements == 0. In addition:
     • Guard SIMD zero padding buffer allocation and workspace size calculation against integer overflow using xnn_safe_mul and xnn_safe_add.
     • Guard scale calculation against division by zero when num_reduction_elements == 0.

2. Subgraph Rank & Axis Reshape Validation:

   • static-constant-pad: Record num_padding_dims in xnn_operator_data and zero-initialize pre/post padding buffers. Validate during reshape that input_value->shape.num_dims >= opdata->num_padding_dims.
   • static-slice: Record num_slice_dims in xnn_operator_data and zero-initialize begins/ends buffers. Validate during reshape that input_value->shape.num_dims == opdata->num_slice_dims.
   • concatenate: Validate axis bounds (axis < 0 || (size_t) axis >= num_dims) early in reshape_concatenate_operator before accessing input shapes or strides. Also fix duplicate word typo in error log ("failed to reshape reshape operator operator").

3. Tests:

   • Added unit tests for zero workspace sizing in batch-matrix-multiply-nc, dynamic-fully-connected-nc, reduce-nd, and resize-bilinear-nhwc.
   • Added subgraph validation tests for invalid axis reshape in concatenate, rank mismatch in static-slice, and input rank less than padding rank in static-constant-pad.