fix: Bedrock provider returning None due to inheritance order

[skamenan7]

Bedrock provider was returning None for both streaming and non-streaming inference, causing 'NoneType' object has no attribute 'choices' errors.

Primary fix: Reorder inheritance to put mixin classes before protocol class in BedrockInferenceAdapter so actual implementations are called.

Additional AWS Bedrock API compatibility fixes:

• Fix non-streaming: use res["body"].read() instead of next(res["body"])
• Fix streaming: add proper event structure checks and safe access
• Disable repetition_penalty (not supported by Bedrock Llama models)

Fixes #3621

Test Plan

Non-Streaming Test

curl -X POST http://localhost:8321/v1/openai/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{"model": "bedrock-inference/meta.llama3-1-8b-instruct-v1:0", "messages": [{"role": "user", "content": "Hello"}], "stream": false}'

Non-Streaming Success Response

{
  "id": "chatcmpl-4a6c063b-53d2-4286-b81d-34ed9c5671e1",
  "choices": [{
    "message": {
      "role": "assistant",
      "content": "Hello. What would you like to talk about?",
      "name": null,
      "tool_calls": null
    },
    "finish_reason": "stop",
    "index": 0,
    "logprobs": null
  }],
  "object": "chat.completion",
  "created": 1759264305,
  "model": "bedrock-inference/meta.llama3-1-8b-instruct-v1:0"
}

Streaming Test

curl -X POST http://localhost:8321/v1/openai/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{"model": "bedrock-inference/meta.llama3-1-8b-instruct-v1:0", "messages": [{"role": "user", "content": "Hello"}], "stream": true}'

Streaming Success Response

data: {"id":"chatcmpl-72161218-0f23-4512-a8bd-042ce5a3dcad","choices":[{"delta":{"content":"","function_call":null,"refusal":null,"role":null,"tool_calls":null},"finish_reason":"stop","index":0,"logprobs":null}],"created":1759264322,"model":"bedrock-inference/meta.llama3-1-8b-instruct-v1:0","object":"chat.completion.chunk","service_tier":null,"system_fingerprint":null,"usage":null}

data: {"id":"chatcmpl-72161218-0f23-4512-a8bd-042ce5a3dcad","choices":[{"delta":{"content":"Hello","function_call":null,"refusal":null,"role":null,"tool_calls":null},"finish_reason":"stop","index":0,"logprobs":null}],"created":1759264322,"model":"bedrock-inference/meta.llama3-1-8b-instruct-v1:0","object":"chat.completion.chunk","service_tier":null,"system_fingerprint":null,"usage":null}

data: {"id":"chatcmpl-72161218-0f23-4512-a8bd-042ce5a3dcad","choices":[{"delta":{"content":". How","function_call":null,"refusal":null,"role":null,"tool_calls":null},"finish_reason":"stop","index":0,"logprobs":null}],"created":1759264322,"model":"bedrock-inference/meta.llama3-1-8b-instruct-v1:0","object":"chat.completion.chunk","service_tier":null,"system_fingerprint":null,"usage":null}

Server Log Snippets - Success

INFO: Found credentials in shared credentials file: ~/.aws/credentials
INFO: BedrockInferenceAdapter.chat_completion [StatusCode.OK] (561.00ms)
INFO: output: {'metrics': None, 'completion_message': {'role': 'assistant', 'content': 'Hello, how can I assist you today?', 'stop_reason': 'end_of_turn', 'tool_calls': []}, 'logprobs': None}
INFO: InferenceRouter.openai_chat_completion [StatusCode.OK] (716.71ms)