feat: add integration testing suite for OpenAI endpoints

[alez007]

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[gemini-code-assist]

Code Review

This pull request introduces a comprehensive integration test suite for the modelship service, covering chat completions, streaming, tool calling, embeddings, and audio processing. It also adds a tool_call_parser field to the TransformersConfig and registers an integration marker in pyproject.toml. Key feedback includes fixing a bug where signal constants were incorrectly accessed via the subprocess module, replacing the deprecated preexec_fn with start_new_session, and improving test robustness by using the tmp_path fixture and relative pathing for configuration files.