refactor(tests): CMake-based CDB, workspace fixture, test cleanup (#378)

## Summary

- **CMake-based CDB generation for module tests**: Replace hand-written
compile_commands.json with CMakeLists.txt (CMake 3.28 `FILE_SET
CXX_MODULES`) in all 26 `tests/data/modules/*/` directories. CDB is
generated on-the-fly via `cmake -G Ninja` during test setup.
- **`@pytest.mark.workspace()` decorator**: Introduce a marker + fixture
pattern so tests declare their workspace via decorator and receive a
resolved `workspace` path. The fixture auto-generates CDB when a
CMakeLists.txt is present.
- **`CliceClient` helper methods**: Add `initialize()`, `open()`,
`wait_diagnostics()`, and `open_and_wait()` to reduce boilerplate across
all test files.
- **Use `asyncio_mode = "auto"`**: Switch from `@pytest_asyncio.fixture`
+ `@pytest.mark.asyncio` to `@pytest.fixture` + auto mode for proper
Pylance type inference on fixtures.
- **Test cleanup**: Remove redundant section separators and docstrings,
delete `tests/pyproject.toml` (config moved to `pytest.ini`).
- **Format task**: Add `.cppm` to `format-cpp` glob pattern.
- **CI fix**: Disable `CMAKE_CXX_SCAN_FOR_MODULES` and prefer pixi
clang++ to fix macOS CI where CMake rejects module scanning.

## Test plan

- [x] All 26 module test directories have CMakeLists.txt with FILE_SET
CXX_MODULES
- [x] generate_cdb() produces valid compile_commands.json with module
flags
- [x] Integration tests pass locally
- [ ] CI passes on all platforms (Linux, macOS, Windows)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Unified fixtures and client workflow: new init/open/wait helpers,
workspace marker support, bounded diagnostics waiting, CMake-based
compilation-database generation, and directory-backed temp-file
workflows; enabled asyncio test mode.
* **Chores**
* Added many C++20 module test projects and test data; removed prior
test pyproject in favor of pytest config; updated formatter to include
.cppm files.
* **Style**
* Reformatted many module/source implementations to consistent
multi-line function bodies.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/data/modules/re_export/CMakeLists.txt

@@ -0,0 +1,13 @@
+cmake_minimum_required(VERSION 3.28)
+project(re_export LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
+add_library(test)
+target_sources(test
+    PUBLIC
+    FILE_SET CXX_MODULES
+    FILES core.cppm wrapper.cppm user.cppm
+)
+

tests/data/modules/re_export/core.cppm

@@ -1,2 +1,5 @@
 export module Core;
-export int core_fn() { return 1; }
+
+export int core_fn() {
+    return 1;
+}

tests/data/modules/re_export/user.cppm

@@ -1,3 +1,6 @@
 export module User;
 import Wrapper;
-export int use_fn() { return core_fn() + wrap_fn(); }
+
+export int use_fn() {
+    return core_fn() + wrap_fn();
+}

tests/data/modules/re_export/wrapper.cppm

@@ -1,3 +1,6 @@
 export module Wrapper;
 export import Core;
-export int wrap_fn() { return core_fn() + 10; }
+
+export int wrap_fn() {
+    return core_fn() + 10;
+}