update prompt caching docs example as caching is now GA for Anthropic

[Sanjan611]

Prompt caching example is outdated since this feature is now GA for Anthropic (and not beta), therefore the beta headers are not required. Also updated the Anthropic model ID as Sonnet 3.5 is retired now.

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Note

Refresh prompt caching docs to use Anthropic GA header and claude-sonnet-4-5-20250929, removing deprecated beta header config.

• Docs (fern/01-guide/05-baml-advanced/prompt-caching.mdx):
  • Update cURL example to use -H "anthropic-version: 2023-06-01" and model claude-sonnet-4-5-20250929.
  • Update BAML client options to model claude-sonnet-4-5-20250929; keep allowed_role_metadata ["cache_control"]; remove beta headers block.
  • Example prompt shows adding cache_control role metadata to messages.

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