fix(postgrest): preserve negation for or filters

[zfaustk]

Summary

not_.or_() currently drops the one-shot negation flag and sends or=(...). This makes the client execute the opposite filter from the requested query. The fix consumes the flag in the shared filter builder and emits not.or=(...), including the reference-table form.

Related issue

Closes #1616

Verification

• uv run --package postgrest pytest -q src/postgrest/tests/_sync/test_filter_request_builder.py src/postgrest/tests/_async/test_filter_request_builder.py src/postgrest/tests/_sync/test_request_builder.py src/postgrest/tests/_async/test_request_builder.py — 176 passed.
• uv run --package postgrest ruff check src/postgrest/src/postgrest/base_request_builder.py src/postgrest/tests/_sync/test_filter_request_builder.py src/postgrest/tests/_async/test_filter_request_builder.py — pass.
• git diff --check — pass.

The focused tests cover ordinary or_(), parent not.or, reference-table cities.not.or, and consumption of the one-shot flag in both sync and async builders.