fix(embedding): skip oversized single item during fallback

[AperturePlus]

Motivation

• Indexing aborted entirely when a single text exceeded the embedding model token limit at minimum batch size, preventing the rest of the codebase from being indexed.
• Design rationale: insert a zero-vector placeholder for permanently oversized items and continue processing to preserve output ordering and indexing continuity; alternatives considered were omitting the item or failing the entire run, but those either break positional mapping or reduce robustness; tradeoff is that downstream consumers must treat zero vectors as placeholders.

Description

• Changed OpenAIEmbeddingClient._embed_with_fallback to log a warning and append a zero-vector ([0.0] * dimension) when a BatchSizeError occurs at min_batch_size, then advance and continue processing.
• Updated client docstrings and exception descriptions to reflect the new behavior for oversized single items.
• Added a property test test_oversized_single_item_is_skipped_with_zero_vector to verify the client continues processing, preserves output ordering, and places a zero-vector at the oversized item's position.

Testing

• Ran uv run ruff check src tests and it passed successfully.
• Ran uv run pytest tests/property/test_embedding_client_properties.py -q and the new test plus file passed.
• Ran uv run pytest tests/ -v --tb=short -q --durations=10 which produced early failures in the full suite unrelated to this change and did not complete cleanly in this execution.
• Ran uv run mypy src --ignore-missing-imports --no-error-summary which reported existing repo-wide type issues unrelated to this change.

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Codex Task