fix: use sys.executable in CLI tests, handle optional dependencies in pptx test, and cleanup youtube converter

[aoright (aoright)]

Summary

1. CLI Tests: Replace hardcoded "python" string in subprocess calls with sys.executable to ensure tests run against the active Python environment.
2. PPTX SVG Test: Add pytest.importorskip for optional dependencies (pptx, lxml) in test_pptx_svg.py to prevent test collection failures when optional extras are not installed.
3. YouTube Converter: Replace redundant dictionary lookup json[k] with v in _findKey(), remove unnecessary pass statement, and raise specific FileConversionException instead of generic Exception.

Validation

• pytest packages/markitdown/tests/test_cli_misc.py packages/markitdown/tests/test_pptx_svg.py -> All tests passed!

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