Fix oinspect TypeError with generic __getattr__ objects; fix test path quoting

oinspect: guard __custom_documentations__ lookup with isinstance(dict) so
objects like polars Expr (which return self for any attribute access via
__getattr__) no longer raise TypeError when inspected with ?. Fixes #15072.

oinspect: fix inspect.Parameter.empty comparison in MIME-hook path; the
previous code accidentally compared against the property descriptor object
rather than the sentinel, making the required-parameter filter a no-op.

test_interactiveshell: wrap sys.executable and self.fname in shlex.quote()
so test_exit_code_signal survives when the source path contains spaces.
Fixes #15100.

https://claude.ai/code/session_01VPnGTgifZ8HMAfiqEVftjj