Add default_headers support to LLM configuration

[max2ma]

Enable custom HTTP headers in OpenAI-compatible API client to support API gateways that use non-standard authentication. Headers support environment variable resolution via ${VAR} syntax.

Changes:

• Add default_headers field to LLMModelConfig
• Resolve environment variables in header values during post-init
• Pass default_headers to OpenAI client when present
• Include default_headers in shared config propagation

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}