feat: add defaultHeaders support to all LLM providers

[marckong]

Add defaultHeaders configuration option to all LLM providers (OpenAI, Anthropic, Azure, Mistral, Llama, Bedrock, Vertex Studio), enabling custom metadata and authentication mechanisms similar to the official OpenAI client.

• Add mergeHeaders utility function for proper header precedence
• Update all provider config types to include defaultHeaders field
• Implement header merging in all fetch calls and SDK initializations
• Priority: required headers > default headers (prevents auth override)

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[subnetmarco]

Needs documentation too.