[Arm/Ethos-U] vela_bin_stream parser does not bounds-check block sizes or IO descriptors

[Arm/Ethos-U] vela_bin_stream parser does not bounds-check block sizes or IO descriptors

🐛 Describe the bug

The Ethos-U runtime parser vela_bin_read() in
backends/arm/runtime/VelaBinStream.cpp reads per-block size fields and the
inputs/outputs descriptor counts and offsets straight from the delegate blob
without validating them against the buffer:

• The per-block size field advances the parse cursor and defines each block's
  payload with no check that it fits in the remaining bytes, so a block can point
  its payload (and the cursor) past the end of the delegate buffer.
• An inputs/outputs block payload is reinterpret_cast to VelaIOs
  ({ int count; VelaIO io[]; }) with no check that the payload is large enough
  to hold count entries.
• At execute time (EthosUBackend.cpp, EthosUBackend_Cortex_M.cpp) the
  io[i].offset and count values from the blob are used to index the scratch
  buffer and the delegate args span with no bound.

The only pre-parse check, vela_bin_validate(), validates alignment and the
vela_bin_stream/vela_end_stream magic strings; it does not inspect any block
size. As a result a malformed or corrupt delegate blob can cause
out-of-bounds reads at model-load time and out-of-bounds access when copying
to/from the Ethos-U scratch buffer at execute time.

This is a memory-safety hardening gap: sibling backends already validate their
delegate blob before use (XNNPACK runs a FlatBuffer verifier, Vulkan bounds-checks
offsets, the Qualcomm delegate was bounds-checked in #22237). The Ethos-U parser
was not covered by that hardening pass.

Affected files

• backends/arm/runtime/VelaBinStream.cpp
• backends/arm/runtime/EthosUBackend.cpp
• backends/arm/runtime/EthosUBackend_Cortex_M.cpp

Versions

Present on main (verified at bf88c643ce); the parser has never had these
bounds checks.

Fix

A fix that adds the missing bounds validation and negative tests is in the linked
PR.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani