fix: allow copied clients to clear default headers and query

[Kayvan-Zahiri]

Passing an empty mapping to copy(set_default_headers={}) or copy(set_default_query={}) currently retains the original defaults. The truthiness fallback treats {} like an omitted replacement. This also affects with_options() and both sync and async clients.

Use an explicit is not None check so an empty replacement clears the selected defaults. Ordinary default_headers / default_query overrides still merge, and None still preserves the inherited values. The change lives in the hand-maintained custom/supermemory/_compat.py; the checked-in src mirror is copied from that source using the same operation as scripts/apply_custom.py.

Added 24 HTTPX MockTransport regression cases exercising actual namespaces.list() requests through both clients and both copy methods. They cover empty/nonempty replacements, omitted replacements, merging, authentication preservation, and the unchanged original client. Twelve cases fail against the original code.

Validation:

• All 63 offline tests pass with Pydantic 2 and separately with Pydantic 1 (pytest --ignore=tests/live -q). No live API calls were made.
• Repository Ruff lint and the CI format check for custom tests scripts examples pass; git diff --check passes.
• Wheel and source distribution build successfully.
• Full mypy still reports the existing generated core/unchecked_base_model.py:158 TypeForm incompatibility; the same error was reproduced after restoring the original compatibility modules.
• Full regeneration was attempted with SKIP_FETCH=1 scripts/generate, but stops because a running Docker daemon is required. The custom source and derived mirror are byte-identical; fresh Fern generation has not been verified locally.