[ENHANCEMENT] Remove deprecated openai-error-handler shim #766

Problem (one or two sentences)

openai-error-handler remains as a deprecated compatibility shim that wraps error-handler, which has been the canonical source since RooCodeInc/Roo-Code#10204. Keeping this shim creates duplicate import paths and duplicate tests for the same handleOpenAIError behavior, increasing maintenance overhead.

Context (who is affected and when)

Provider and embedder code that uses OpenAI-compatible error handling currently has references to the deprecated openai-error-handler shim, while the actual canonical implementation lives in error-handler.

This can make it unclear which module should be used when updating or reviewing error handling logic. It also leaves deprecated code and duplicate tests in the codebase even though the canonical implementation already exists.

Desired behavior (conceptual, not technical)

OpenAI-compatible error handling should be managed through a single canonical utility: error-handler.

The deprecated openai-error-handler shim and its duplicate test file should be removed, and all related imports should reference error-handler directly. The existing handleOpenAIError behavior should remain unchanged.

Constraints / preferences (optional)

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Request checklist

• I've searched existing Issues and Discussions for duplicates
• This describes a specific problem with clear context and impact

Zoo Code Task Links (optional)

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Acceptance criteria (optional)

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Proposed approach (optional)

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Trade-offs / risks (optional)

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