GH-49697: [C++][CI] Check IPC file body bounds are in sync with decoder outcome

[pitrou]

Rationale for this change

When we read an IPC message from an IPC file, we validate its associated body size against the amount required by the streaming decoder. However, we're currently only checking that the body size is large enough, not that it's exactly as expected.

An invalid IPC file might advertise in its footer a metaDataLength that's larger than the actual serialized Flatbuffers payload. In that case, the associated body would start before the offset computed from the IPC file footer.

What changes are included in this PR?

Strengthen body size check against expected decoder read, to ensure that the metadata length advertised in the IPC file footer is consistent with the actual size of the Flatbuffers-serialized metadata.

Are these changes tested?

By additional fuzz regression file.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Check IPC file body bounds are in sync with decoder outcome #49697

[pitrou]

@github-actions crossbow submit -g cpp

[github-actions]

Revision: 44711e3

Submitted crossbow builds: ursacomputing/crossbow @ actions-cc0beb58ed

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency
example-cpp-tutorial
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-valgrind
test-debian-13-cpp-amd64
test-debian-13-cpp-i386
test-debian-experimental-cpp-gcc-15
test-fedora-42-cpp
test-ubuntu-22.04-cpp
test-ubuntu-22.04-cpp-20
test-ubuntu-22.04-cpp-bundled
test-ubuntu-22.04-cpp-emscripten
test-ubuntu-22.04-cpp-no-threading
test-ubuntu-24.04-cpp
test-ubuntu-24.04-cpp-bundled-offline
test-ubuntu-24.04-cpp-gcc-13-bundled
test-ubuntu-24.04-cpp-gcc-14
test-ubuntu-24.04-cpp-minimal-with-formats
test-ubuntu-24.04-cpp-thread-sanitizer