Fix oinspect TypeError with objects using generic __getattr__

Objects whose __getattr__ returns something other than dict for __custom_documentations__ (e.g. polars Expr, which returns a new Expr for any attribute name) no longer cause a TypeError when inspected with ? or %pinfo(). The lookup is now guarded with isinstance(..., dict). #15072

Also fixed an incorrect comparison in the MIME-hook inspection path (inspect.Parameter.default → inspect.Parameter.empty) that accidentally relied on a property-object inequality to filter required parameters.

Fix test failures when IPython source path contains spaces

test_exit_code_signal in tests/test_interactiveshell.py now uses shlex.quote() when interpolating sys.executable and the temporary filename into a shell command string, preventing test failures on systems where either path contains spaces. #15100