…ential leakage

• Added customrepr methods to ChatOpenAI, BaseOpenAI, and OpenAIEmbeddings classes
• API keys are now redacted showing only first 3 and last 4 characters (e.g., 'sk-...wxyz')
• Short keys (< 8 chars) are completely masked with '***'
• Callable API key providers are also properly masked
• Added comprehensive unit tests to verify the fix

Fixes#34356

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