[codex] Use non-view registry for Python register

[jprafael]

Summary

• replace the Python register() implementation with a connection-local registry resolved through replacement scans instead of creating a temporary view
• make unregister() erase the registered Python dependency immediately, including inside an open transaction
• keep existing temp object conflict behavior and add a pandas regression test that verifies the DataFrame can be collected after unregistering inside a transaction

Root Cause

register() created a temporary view with an external dependency on the Python object. When unregister() ran inside a transaction, DROP VIEW did not release that dependency until the transaction completed, so large DataFrames stayed alive across the transaction.

Validation

• SKBUILD_EDITABLE_SKIP=/home/ubuntu/duckdb-sqlite-worktrees/duckdb-python-v1.5.3/build/debug python -m pytest tests/fast/pandas/test_pandas_unregister.py tests/fast/arrow/test_unregister.py tests/fast/api/test_duckdb_connection.py::TestDuckDBConnection::test_register_relation tests/fast/api/test_duckdb_connection.py::TestDuckDBConnection::test_unregister_problematic_behavior tests/fast/pandas/test_same_name.py::TestMultipleColumnsSameName::test_multiple_columns_with_same_name -q
• manual weakref check: unregister() inside a transaction releases the DataFrame immediately and rollback does not resurrect the registered name
• full-size chunk sanity check: RSS growth still reproduces through the insert path without register(), while each registered DataFrame weakref is released after unregister()