chore: parallelize cpp integration tests

[fresh-borzoni]

Summary

closes #373
Parallelize C++ integration tests using CTest, matching the Rust test model.

Each test runs as a separate process via gtest_discover_tests().
The first process starts the Docker cluster, others detect it via port check and reuse it.

[fresh-borzoni]

@luoyuxia @leekeiabstraction @charlesdong1991 PTAL 🙏

[Copilot]

Pull request overview

This PR parallelizes the C++ integration tests by switching from a single add_test() invocation to CTest-driven per-test execution via gtest_discover_tests(), and updates the Docker-backed Fluss test cluster lifecycle so multiple test processes can share a single cluster instance.

Changes:

• Discover and register each GoogleTest case as an individual CTest test to enable ctest -j parallelism.
• Add a CTest fixtures cleanup hook (--cleanup) to stop Docker resources after the suite completes.
• Update cluster startup logic to reuse an already-running local cluster (or one started by another parallel test process).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
bindings/cpp/test/test_utils.h	Adds cluster reuse/“wait for other process” behavior, refactors bootstrap server setup, and changes teardown behavior.
bindings/cpp/test/test_main.cpp	Adds --cleanup flag handling to stop Docker resources on fixture cleanup runs.
bindings/cpp/CMakeLists.txt	Switches to gtest_discover_tests() and adds a FIXTURES_CLEANUP test for Docker cleanup.
.github/workflows/build_and_test_cpp.yml	Runs C++ integration tests in parallel via ctest -j$(nproc).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[fresh-borzoni]

Answered copilot comments

[luoyuxia]

@fresh-borzoni Thanks for the pr. LGTM!