Return all embeddings for batched OpenAI inputs

[YAshhh29]

Why

The OpenAI Embeddings API accepts batches, but OpenAIHook.create_embeddings() returned only response.data[0]. Every successful batch result after the first item was silently discarded.

What changed

• Preserve the existing single-input return shape.
• Return every batch embedding ordered by the response index.
• Update the operator return type and user documentation.
• Add regressions for string and token-array batches.

Tests

• Focused batch regression test against this branch: 2 passed.
• The same regression test against the unmodified implementation: 2 failed.
• ruff format and ruff check --fix: passed.
• git diff --check: passed.

The full provider suite was not run locally because Airflow does not support native Windows; CI provides the Linux suite result.

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Was generative AI tooling used to co-author this PR?

• Yes - GitHub Copilot

Generated-by: GitHub Copilot following the guidelines

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Drafted-by: GitHub Copilot (no human review before posting)