fix(embedding): omit default OpenAI dimensions

[Premsenareddy]

Summary

• make OpenAI embedding dimensions optional at runtime so OpenAI-compatible providers can use their native defaults
• only send the dimensions parameter when explicitly configured
• keep YAML model-card defaults rendering as concrete integer schemas for built-in OpenAI models

Fixes #1831

Tests

• .venv/bin/python -m pytest tests/embedding_openai_test.py -q
• .venv/bin/python -m pytest tests/embedding_openai_test.py tests/embedding_dashscope_test.py tests/embedding_gemini_test.py tests/embedding_ollama_test.py -q -p no:cacheprovider
• .venv/bin/pre-commit run --files src/agentscope/embedding/_embedding_base.py src/agentscope/embedding/_embedding_model_card.py src/agentscope/embedding/_openai/_model.py tests/embedding_openai_test.py