tests: rb: Add type conversion test for ruby loader

[AshutoshSharma-pixel]

Adds metacall_ruby_conversion_test, modeled after metacall_node_conversion_test.

The test loads an identity function from memory using the rb loader and runs it
against all METACALL_* types, printing the conversion matrix.

Local test output on macOS with Ruby 3.2.2:

Boolean => Boolean
Fixnum => Fixnum
Bignum => Bignum
Float => Float
String => String
Array => Array
NilClass => NilClass

Notable difference from the Node loader: Ruby preserves numeric type granularity
(Fixnum, Bignum, Float remain distinct) whereas Node collapses all numeric types
to METACALL_DOUBLE.

Tested locally with:
cmake -DOPTION_BUILD_LOADERS=ON -DOPTION_BUILD_LOADERS_RB=ON -DOPTION_BUILD_TESTS=ON
cmake --build . --target metacall-ruby-conversion-test
ctest -R metacall-ruby-conversion-test --verbose

[ PASSED ] 1 test

[viferga]

Running CI

[viferga]

Add tests please.

[AshutoshSharma-pixel]

added assertions for Bool, Int, Long, Double, String, Array, Map and Buffer round-trips. also implemented Buffer (IO::Buffer), Array and Map serialization/deserialization in rb_loader. all pass locally. [ PASSED ] 1 test

[AshutoshSharma-pixel]

fixed the ubuntu:jammy build failure ruby/io/buffer.h is only available in Ruby >= 3.0, so I've gated the IO::Buffer include and usage behind RUBY_API_VERSION_MAJOR >= 3. on older Ruby versions the buffer falls back to a plain binary string. also added ruby/version.h to the loader include header to ensure the macro is defined. still passes locally.