test: cover editable playground includes

[coyaSONG]

Summary

• extend the existing playground include fixture with the editable form
• assert that the rendered code block preserves the editable class

This covers one of the remaining integration-test cases listed in #2677 without changing runtime behavior.

Validation

• red proof: the targeted test failed after adding the fixture because the snapshot did not contain the editable heading and language-rust editable class
• cargo test --test testsuite includes::playground_include -- --exact
• cargo test --test testsuite
• cargo test --workspace
• cargo test --workspace --no-default-features
• cargo clippy --workspace --all-targets --no-deps -- -D warnings
• cargo fmt --all --check
• git diff --check

Disclosure

This change was prepared with OpenAI Codex assistance and manually reviewed and validated with the commands above.