CLI: Optimize generate_clients.py with dirs pruning

[MonkeyCanCode]

This PR primary optimizes generate_clients.py with dirs pruning by using os.walk as oppose to rglob. The previous code scanned up to 6.4k files path and the updated one will scanned only ~250.

Here is the old output (in verbose mode):

$ python3 generate_clients.py -v
...
test/test_privileges_command.py: skipped (path excluded)
...
templates/header-md.txt: skipped (path excluded)
...
.venv/bin/activate.bat: skipped (path excluded)
...
.venv/lib/python3.11/site-packages/pyroaring.cpython-311-darwin.so: skipped (path excluded)
...
apache_polaris/sdk/catalog/__pycache__/rest.cpython-311.pyc: skipped (extension excluded)
...
apache_polaris/cli/command/__pycache__/__init__.cpython-313.pyc: skipped (extension excluded)

And the new output (in verbose mode) which we skipped a lot of those no-ops dirs/files:

$ python3 generate_clients.py -v
...
apache_polaris/sdk/__init__.py: updated
...
apache_polaris/sdk/management/api/polaris_default_api.py: updated
...

Beside the above change, this PR also addresses couple NITs:

1. Change dir from test to tests as there is more than one test file now
2. Skip test dir cleanup logic as that is no longer needed since f0ee460 (as I added --global-property=apiDocs=false,modelDocs=false,modelTests=false,apiTests=false
3. Cleanup .openapi-generator-ignore as bit to remove default comments and non-existed files

Checklist

• 🛡️ Don't disclose security issues! (contact security@apache.org)
• 🔗 Clearly explained why the changes are needed, or linked related issues: Fixes #
• 🧪 Added/updated tests with good coverage, or manually tested (and explained how)
• 💡 Added comments for complex logic
• 🧾 Updated CHANGELOG.md (if needed)
• 📚 Updated documentation in site/content/in-dev/unreleased (if needed)