gguf-py: validate n_dims and guard against uint64 overflow in reader (#25401)

The Python GGUF reader lacked two guards the C++ loader has:
- n_dims read as uint32 with no GGML_MAX_DIMS bound -> crafted file with
  huge n_dims triggers oversized memmap read / OOM.
- np.prod(dims) on uint64 wraps silently -> a crafted dims triple can
  overflow to a tiny element count, passing an undersized read through.

Add a GGML_MAX_DIMS check and compute the element count with Python ints.

Fixes #25378

gguf-py/gguf/constants.py

@@ -11,6 +11,7 @@ GGUF_MAGIC             = 0x46554747  # "GGUF"
 GGUF_VERSION           = 3
 GGUF_DEFAULT_ALIGNMENT = 32
 GGML_QUANT_VERSION     = 2  # GGML_QNT_VERSION from ggml.h
+GGML_MAX_DIMS          = 4  # GGML_MAX_DIMS from ggml.h
 
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