fix(stream): preserve non-negative tool call indexes

[bokmann]

Summary

• fix OpenAI-compatible streaming tool-call indexes so they stay non-negative
• map Bedrock contentBlockIndex values to zero-based tool-call ordinals instead of subtracting 1
• add a regression test covering the first streamed tool call and multiple tool calls in one stream

Problem

Streaming tool-call chunks were being emitted with tool_calls[].index = -1 for the first tool call. Strict OpenAI-compatible clients reject that shape and fail during streamed tool use.

I reproduced this against a live deployment with a request like:

{
  "model": "us.anthropic.claude-sonnet-4-6",
  "stream": true,
  "stream_options": {"include_usage": true},
  "messages": [{"role": "user", "content": "Use a tool call. Return weather for Boston."}],
  "tools": [{
    "type": "function",
    "function": {
      "name": "get_weather",
      "description": "Get weather",
      "parameters": {
        "type": "object",
        "properties": {"city": {"type": "string"}},
        "required": ["city"]
      }
    }
  }]
}

Before this patch, the stream contained chunks like:

{"delta":{"tool_calls":[{"index":-1,"id":"tooluse_...","type":"function","function":{"name":"get_weather","arguments":""}}]}}

That matches the existing negative-index bug report in #203.

Fix

The converter was using contentBlockIndex - 1 as the OpenAI tool_calls[].index. That is not safe, because contentBlockIndex is a content-block position, not a tool-call ordinal.

This patch keeps a per-stream mapping from Bedrock contentBlockIndex to the next zero-based tool-call ordinal and reuses that mapping for later deltas from the same tool call.

Validation

Ran the focused regression test inside the project runtime image:

docker run --rm \
  -e PYTHONPATH=/work/src \
  -v /tmp/bedrock-access-gateway-bokmann:/work \
  -w /work \
  --entrypoint python \
  919493069736.dkr.ecr.us-east-1.amazonaws.com/bedrock-proxy-api-fabro@sha256:2ef96c1248daee22db1eee404f3fe15e55d728dc937251730e4b64a1e41f5f4e \
  -m unittest tests.test_bedrock_stream_tool_call_index -v

Fixes #203.