Fix invalid parallel_tool_calls field in Anthropic requests (#11)

[benwoody]

Fixes #11.

Problem

The Claude (OAuth) provider was failing every request with HTTP 400:

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "parallel_tool_calls: Extra inputs are not permitted"
  },
  "request_id": "<...>"
}

AnthropicClient.serializeRequest was unconditionally adding parallel_tool_calls to every Anthropic request whenever tools was non-null. That field belongs to OpenAI's Chat Completions API — Anthropic's
Messages API rejects it as an unknown input.

Fix

Translate to Anthropic's equivalent: tool_choice.disable_parallel_tool_use (inverted). Only set when the user has opted out of parallel tool calls, so the default (parallel allowed) stays implicit in
the request body.

  request.tools?.let { tools ->
      put("tools", kotlinx.serialization.json.JsonArray(tools))
      if (!request.parallelToolCalls) {
          put("tool_choice", kotlinx.serialization.json.buildJsonObject {
              put("type", kotlinx.serialization.json.JsonPrimitive("auto"))
              put("disable_parallel_tool_use", kotlinx.serialization.json.JsonPrimitive(true))
          })
      }
  }

This preserves the existing Budget Mode "parallel tool calls" toggle behavior — when the user disables parallel tools in the UI, it now correctly translates to the Anthropic API's serial-tool-use
directive instead of being silently malformed.

Notes

• OpenAiFormatAdapter.kt deliberately untouched. It also calls put("parallel_tool_calls", ...) but that field is valid in OpenAI-format requests — only the Anthropic client was wrong.
• No test added. AnthropicClient has no existing test coverage and the repo doesn't yet use MockWebServer-style HTTP testing. Happy to follow up with a small test (and bump serializeRequest from
  private to internal) if you'd prefer.

Verified

Tested locally on a dGEN1 with the Claude (OAuth) provider and claude-sonnet-4-6: tool-using requests now complete instead of failing at body validation.