refactor(tests): reorganize integration tests into domain-based subdirectories (#409)

## Summary
- Extract shared test utilities into `tests/integration/utils/` (client,
workspace, assertions, wait, cache)
- Migrate 12 test files into categorized subdirectories: `lifecycle/`,
`compilation/`, `features/`, `modules/`, `extensions/`, `stress/`
- Merge `test_include_completion.py` + `test_import_completion.py` →
`features/test_completion.py`
- Remove stale directory-tree comments and section divider comments

## Test plan
- [x] `pytest --collect-only` collects all 113 tests
- [x] All test module imports verified
- [x] `pixi run format` applied

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* **Tests**
* Reorganized integration suites: added new feature tests (completion,
server) and removed older duplicated modules.
* Centralized shared test utilities and assertion helpers for
diagnostics, workspace operations, waiting/synchronization, and cache
inspection.
* **Chores / Refactor**
* Standardized test client lifecycle and helper usage across suites for
more reliable test flows.
* Improved server session lifecycle handling for more predictable
document/session resets during tests.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

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

Author: 16bit-ykiko

src/server/master_server.cpp

@@ -308,8 +308,12 @@ void MasterServer::register_handlers() {
         auto path = uri_to_path(params.text_document.uri);
         auto path_id = workspace.path_pool.intern(path);
 
-        auto [it, _] = sessions.try_emplace(path_id);
+        auto [it, inserted] = sessions.try_emplace(path_id);
         auto& session = it->second;
+        if(!inserted) {
+            // DenseMap tombstone may retain stale data — reset to a fresh Session.
+            session = Session{};
+        }
         session.path_id = path_id;
         session.version = params.text_document.version;
         session.text = params.text_document.text;