feat(amazon-bedrock): forward Anthropic beta flags via additionalMode…

[adhajar-amzn]

…lRequestFields

Bedrock Converse ignores the anthropic-beta HTTP header that the direct Anthropic API honors. The Bedrock-correct channel for opting into Anthropic experimental features is the top-level
additionalModelRequestFields.anthropic_beta array on the ConverseStream request, which Bedrock forwards to the underlying Anthropic model.

This exposes a new anthropicBetas.betas plugin config that, when set, wraps the stream function for Anthropic-family Bedrock models and merges the configured beta flags into the outbound request body. The wrapper is a no-op for non-Anthropic families (Nova, Titan, Llama, etc.), which would reject the unknown field.

Motivation: enabling fine-grained-tool-streaming-2025-05-14 so that large tool_use input JSON (e.g. multi-KB heredoc scripts) streams incrementally via input_json_delta events instead of being buffered until the tool_use block is complete. Users observed 60-80s of silence on turns where the model generates large tool_use arguments.

Implementation mirrors the existing createGuardrailWrapStreamFn pattern and composes cleanly with guardrail injection.

Example config:

{
"plugins": {
"entries": {
"amazon-bedrock": {
"config": {
"anthropicBetas": {
"betas": ["fine-grained-tool-streaming-2025-05-14"]
}
}
}
}
}
}

Summary

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If this PR fixes a plugin beta-release blocker, title it fix(<plugin-id>): beta blocker - <summary> and link the matching Beta blocker: <plugin-name> - <summary> issue labeled beta-blocker. Contributors cannot label PRs, so the title is the PR-side signal for maintainers and automation.

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