An upcoming change switches compress() to emitting DECODE-based
models. The Python ops resolver registers the DECODE operator
unconditionally, so those models load successfully even in a build
without compression support. That breaks this test's original
approach, which ran a model through compress() and expected the load
to fail. Rewrite it to instead inject a raw COMPRESSION_METADATA
entry into the flatbuffer via model_editor, directly exercising the
HasCompressionMetadata() detection path for legacy-compressed models.
Decoupling the test from compress() output lets it verify the
legacy-rejection behavior independently of what compress() emits, so
it passes both before and after the switch.
BUG=part of #3256
When a model contains COMPRESSION_METADATA but the interpreter was built
without compression support, throw a RuntimeError with a helpful message
directing users to build with --//:with_compression=true.
The implementation uses inline functions in compression_utils.h
that are optimized away when compression is disabled, ensuring
all code paths remain compile-checked, and readable without
preprocessor clutter.
Includes test_compression_unsupported.py to verify the error detection,
which only runs when compression is disabled.
BUG=#3125
