fix: handle OSError in _get_mtime for non-file paths

[gaoflow]

_get_mtime() calls os.path.getmtime() on any non-remote path,
but some paths that look like local filesystem paths are actually
GDAL virtual filesystems (/vsicurl/..., /vsis3/...) or other
URI schemes that don't support stat(). This causes OSError to
propagate and crash open_dataset().

Wrap the getmtime call in contextlib.suppress(OSError) so that
non-file paths gracefully return mtime = None instead of raising.

• Tests added
• All tests pass

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[wahajahmed010]

test

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