internal: add safety comments in pyclass/gc, call and marshal

[xavierforge]

Part of #5487, continuing from #6256, #6259 and #6265.

Removes the #![allow(clippy::undocumented_unsafe_blocks)] exemption from three files and adds // SAFETY: comments for their five unsafe blocks:

• src/pyclass/gc.rs
• src/call.rs
• src/marshal.rs

One commit per file for easier review. Verified with clippy under both the default and abi3 feature sets to cover the Py_LIMITED_API branches (note marshal.rs is cfg-gated out under Py_LIMITED_API, so the abi3 pass does not exercise it).

Notes for review: the pyclass/gc.rs argument leans on PyVisit's construction invariant (only created by PyO3's tp_traverse implementation, cannot outlive that call). The PyMarshal_WriteObjectToString comment deliberately claims only that a non-NULL return is a new owned reference to a bytes object: unlike the read direction, its C API documentation does not document the error return, and NULL is permitted by Bound::from_owned_ptr_or_err regardless.

Disclosure: I used AI assistance while analyzing this code; I have personally verified each safety argument against the documented contracts.