fix(anthropic): send tool_choice none instead of stripping tools

[roaminro]

Summary

• When toolChoice is 'none', send tool_choice: { type: "none" } with the tools array preserved instead of stripping both tools and tool_choice from the API request
• Add { type: 'none' } to the AnthropicToolChoice type union
• Update test to expect tools and toolChoice to be preserved

Problem

When toolChoice is 'none', the provider strips both tools and tool_choice from the request:

case 'none':
  // Anthropic does not support 'none' tool choice, so we remove the tools:
  return { tools: undefined, toolChoice: undefined, toolWarnings, betas };

This causes the Anthropic API to return content: [] (empty response) when the conversation history contains tool_use/tool_result blocks, because it sees tool blocks in history but no tools parameter.

This is a common issue in agentic loops where the last step needs to force a text-only response — the model produces zero text instead of a summary.

Anthropic API support

The comment "Anthropic does not support 'none' tool choice" is outdated. The Anthropic API officially supports tool_choice: { type: "none" } — it's listed in the tool use pricing table for every model from Claude 3 through Claude Opus 4.6:

Model	Tool choice	Token count
Claude Opus 4.6	auto, none	346 tokens

The docs also note: "If no tools are provided, then a tool choice of none uses 0 additional system prompt tokens."

Fixes #12378

Test plan

• Updated existing should handle tool choice "none" test to verify tools and toolChoice are preserved
• All 42 existing prepareTools tests pass