Add C++23 deducing this tests (test_cpp23features.py)

[conrade-ctc]

Exercise C++23 explicit object parameters (P0847R7, "deducing this") end-to-end through the interpreter (clang-repl and cling).

C++23 isn't the stack default and the interpreter is a process-wide singleton pinned at first import cppyy, so TestCPP23Driver re-runs the class once in a child process that selects C++23 before import; all tests share that one interpreter. Bodies are ordinary Python; only the cppdef C++ source is a string.

Covers the language mechanics (basic, const, by-value, extra-args, rvalue-ref, chaining, default arg, call operator, mixed overload, templated, abbreviated this auto&&, inheritance) and the use-cases from the MSVC "Deducing this" blog post.

Depends on CppInterOp PR compiler-research/CppInterOp#1032 (merge that first).

---

🤖 Done with the help of Claude Code (Claude Opus 4.8)

[conrade-ctc]

@Vipul-Cariappa @aaronj0 @vgvassilev this looks green now with the other PR landing for CppInterOp, this is just new test to exercise the deducing this from cppyy side

[vgvassilev]

LGTM!

[Vipul-Cariappa]

The CI logs say:

test_cpp23features.py::TestCPP23FEATURES::test01_deducing_this_basic SKIPPED [ 13%]
test_cpp23features.py::TestCPP23FEATURES::test02_deducing_this_const SKIPPED [ 13%]
test_cpp23features.py::TestCPP23FEATURES::test03_deducing_this_by_value SKIPPED [ 13%]
test_cpp23features.py::TestCPP23FEATURES::test04_deducing_this_with_extra_args SKIPPED [ 13%]
test_cpp23features.py::TestCPP23FEATURES::test05_deducing_this_rvalue_ref SKIPPED [ 14%]
test_cpp23features.py::TestCPP23FEATURES::test05b_traditional_rvalue_ref_qualifier SKIPPED [ 14%]
test_cpp23features.py::TestCPP23FEATURES::test06_deducing_this_chaining SKIPPED [ 14%]
test_cpp23features.py::TestCPP23FEATURES::test07_deducing_this_default_arg SKIPPED [ 14%]
test_cpp23features.py::TestCPP23FEATURES::test08_deducing_this_call_operator SKIPPED [ 14%]
test_cpp23features.py::TestCPP23FEATURES::test09_deducing_this_mixed_overload SKIPPED [ 14%]
test_cpp23features.py::TestCPP23FEATURES::test10_deducing_this_templated SKIPPED [ 15%]
test_cpp23features.py::TestCPP23FEATURES::test11_deducing_this_templated_with_args SKIPPED [ 15%]
test_cpp23features.py::TestCPP23FEATURES::test12_deducing_this_templated_returns_self_type SKIPPED [ 15%]
test_cpp23features.py::TestCPP23FEATURES::test13_deducing_this_abbreviated_auto SKIPPED [ 15%]
test_cpp23features.py::TestCPP23FEATURES::test14_deducing_this_by_value_copy_semantics SKIPPED [ 15%]
test_cpp23features.py::TestCPP23FEATURES::test15_deducing_this_inheritance SKIPPED [ 15%]
test_cpp23features.py::TestCPP23FEATURES::test16_blog_deduplication SKIPPED [ 16%]
test_cpp23features.py::TestCPP23FEATURES::test17_blog_crtp_postfix_increment SKIPPED [ 16%]
test_cpp23features.py::TestCPP23FEATURES::test18_blog_recursive_lambda SKIPPED [ 16%]
test_cpp23features.py::TestCPP23FEATURES::test19_blog_lambda_forwarding SKIPPED [ 16%]
test_cpp23features.py::TestCPP23FEATURES::test20_blog_pass_by_value SKIPPED [ 16%]
test_cpp23features.py::TestCPP23FEATURES::test21_blog_sfinae_friendly_transform SKIPPED [ 16%]
test_cpp23features.py::TestCPP23Driver::test_run_under_cpp23 PASSED      [ 17%]

Many skips with one pass. The reader needs to know the implementation to figure out that all tests actually passed.
Right now, we do not have any means to configure the interpreter arguments programmatically. We might want to figure this out.

@vgvassilev, this might be a use case for sub-interpreters.

LGTM, otherwise. We can merge it.