* Fix open router prompt caching with messages API
* Send anthropic-beta headers for OpenRouter Messages API
Extract getAnthropicBetaHeader() on ChatEndpoint so subclasses that
override getExtraHeaders() (like OpenAIEndpoint) can opt in. Override
it on OpenRouterEndpoint so anthropic-beta (interleaved-thinking,
advanced-tool-use, context-management) is sent on the /messages path.
* Use TokenizerType.O200K instead of 'as any' in OpenRouter test
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Co-authored-by: bhavyaus <bhavyau@microsoft.com>
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