Fix bfbs Lua/Nim generators: honor -o and create namespace dirs

[beingbrijesh]

"Fixes #8762 ": Use ConCatPathFileName(options_.output_path, path) and EnsureDirExists(full_dir) so generators create namespace directories under the provided -o path.

Also: added goldens/schema/namespace_first.fbs, updated Lua goldens, added tests/test_bfbs_namespace_output.py (Lua + Nim tests), and integrated the Python test into CMake.

Testing: built flatc, ran CTest; both new tests passed locally.

[beingbrijesh]

@cosmith-nvidia @dbaileychess @aardappel can you please review my fixes for the issue #8762

[jtdavis777]

@beingbrijesh just a friendly note - if you update your description with "fixes #8762" it will link this PR and that issue :)

[beingbrijesh]

@beingbrijesh just a friendly note - if you update your description with "fixes #8762" it will link this PR and that issue :)

@thejtshow thank you for your feedback, I have updated the description :)

[aardappel]

This is not the way to do it, the goldens dir is for a specific purpose and we should not add new schemas there that are only generated for 2 languages.

It also generates new files (for Nim) without renaming the old ones.

Generally, if something can be tested with the existing files in tests that is better.