gguf : harden loader against malformed tensor dims and metadata types (llama/25596)

gguf : harden loader against malformed tensor dims and metadata types (llama/25596)

* gguf : harden loader against malformed tensor dims and metadata types

* gguf: address review on malformed-metadata hardening

- report the expected vs. actual type when general.alignment is not u32
- use ggml_nelements() > 0 for the zero-element guard and keep the
  representability checks visually aligned
- add test-gguf cases for a wrong-typed alignment key and a zero-dim
  tensor (both used to crash: assert-abort and SIGFPE respectively)

Ran tests/test-gguf: 164/164 pass. Used an AI assistant to help draft
these edits; reviewed and verified by me.

* cont : less comments

Co-authored-by: Georgi Gerganov <redacted>
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Co-authored-by: Georgi Gerganov <redacted>