feat: add OpenAI-compatible Bedrock provider (#3748)

Implements AWS Bedrock inference provider using OpenAI-compatible
endpoint for Llama models available through Bedrock.

Closes: #3410


## What does this PR do?

Adds AWS Bedrock as an inference provider using the OpenAI-compatible
endpoint. This lets us use Bedrock models (GPT-OSS, Llama) through the
standard llama-stack inference API.

The implementation uses LiteLLM's OpenAI client under the hood, so it
gets all the OpenAI compatibility features. The provider handles
per-request API key overrides via headers.

## Test Plan

**Tested the following scenarios:**
- Non-streaming completion - basic request/response flow
- Streaming completion - SSE streaming with chunked responses
- Multi-turn conversations - context retention across turns
- Tool calling - function calling with proper tool_calls format

# Bedrock OpenAI-Compatible Provider - Test Results


**Model:** `bedrock-inference/openai.gpt-oss-20b-1:0`


---

## Test 1: Model Listing

**Request:**
```http
GET /v1/models HTTP/1.1
```

**Response:**
```http
HTTP/1.1 200 OK
Content-Type: application/json

{
  "data": [
    {"identifier": "bedrock-inference/openai.gpt-oss-20b-1:0", ...},
    {"identifier": "bedrock-inference/openai.gpt-oss-40b-1:0", ...}
  ]
}
```

---

## Test 2: Non-Streaming Completion

**Request:**
```http
POST /v1/chat/completions HTTP/1.1
Content-Type: application/json

{
  "model": "bedrock-inference/openai.gpt-oss-20b-1:0",
  "messages": [{"role": "user", "content": "Say 'Hello from Bedrock' and nothing else"}],
  "stream": false
}
```

**Response:**
```http
HTTP/1.1 200 OK
Content-Type: application/json

{
  "choices": [{
    "finish_reason": "stop",
    "message": {"content": "...Hello from Bedrock"}
  }],
  "usage": {"prompt_tokens": 79, "completion_tokens": 50, "total_tokens": 129}
}
```

---

## Test 3: Streaming Completion

**Request:**
```http
POST /v1/chat/completions HTTP/1.1
Content-Type: application/json

{
  "model": "bedrock-inference/openai.gpt-oss-20b-1:0",
  "messages": [{"role": "user", "content": "Count from 1 to 5"}],
  "stream": true
}
```

**Response:**
```http
HTTP/1.1 200 OK
Content-Type: text/event-stream

[6 SSE chunks received]
Final content: "1, 2, 3, 4, 5"
```

---

## Test 4: Error Handling - Invalid Model

**Request:**
```http
POST /v1/chat/completions HTTP/1.1
Content-Type: application/json

{
  "model": "invalid-model-id",
  "messages": [{"role": "user", "content": "Hello"}],
  "stream": false
}
```

**Response:**
```http
HTTP/1.1 404 Not Found
Content-Type: application/json

{
  "detail": "Model 'invalid-model-id' not found. Use 'client.models.list()' to list available Models."
}
```

---

## Test 5: Multi-Turn Conversation

**Request 1:**
```http
POST /v1/chat/completions HTTP/1.1

{
  "messages": [{"role": "user", "content": "My name is Alice"}]
}
```

**Response 1:**
```http
HTTP/1.1 200 OK

{
  "choices": [{
    "message": {"content": "...Nice to meet you, Alice! How can I help you today?"}
  }]
}
```

**Request 2 (with history):**
```http
POST /v1/chat/completions HTTP/1.1

{
  "messages": [
    {"role": "user", "content": "My name is Alice"},
    {"role": "assistant", "content": "...Nice to meet you, Alice!..."},
    {"role": "user", "content": "What is my name?"}
  ]
}
```

**Response 2:**
```http
HTTP/1.1 200 OK

{
  "choices": [{
    "message": {"content": "...Your name is Alice."}
  }],
  "usage": {"prompt_tokens": 183, "completion_tokens": 42}
}
```

**Context retained across turns**

---

## Test 6: System Messages

**Request:**
```http
POST /v1/chat/completions HTTP/1.1

{
  "messages": [
    {"role": "system", "content": "You are Shakespeare. Respond only in Shakespearean English."},
    {"role": "user", "content": "Tell me about the weather"}
  ]
}
```

**Response:**
```http
HTTP/1.1 200 OK

{
  "choices": [{
    "message": {"content": "Lo! I heed thy request..."}
  }],
  "usage": {"completion_tokens": 813}
}
```


---

## Test 7: Tool Calling

**Request:**
```http
POST /v1/chat/completions HTTP/1.1

{
  "messages": [{"role": "user", "content": "What's the weather in San Francisco?"}],
  "tools": [{
    "type": "function",
    "function": {
      "name": "get_weather",
      "parameters": {"type": "object", "properties": {"location": {"type": "string"}}}
    }
  }]
}
```

**Response:**
```http
HTTP/1.1 200 OK

{
  "choices": [{
    "finish_reason": "tool_calls",
    "message": {
      "tool_calls": [{
        "function": {"name": "get_weather", "arguments": "{\"location\":\"San Francisco\"}"}
      }]
    }
  }]
}
```

---

## Test 8: Sampling Parameters

**Request:**
```http
POST /v1/chat/completions HTTP/1.1

{
  "messages": [{"role": "user", "content": "Say hello"}],
  "temperature": 0.7,
  "top_p": 0.9
}
```

**Response:**
```http
HTTP/1.1 200 OK

{
  "choices": [{
    "message": {"content": "...Hello! 👋 How can I help you today?"}
  }]
}
```

---

## Test 9: Authentication Error Handling

### Subtest A: Invalid API Key

**Request:**
```http
POST /v1/chat/completions HTTP/1.1
x-llamastack-provider-data: {"aws_bedrock_api_key": "invalid-fake-key-12345"}

{"model": "bedrock-inference/openai.gpt-oss-20b-1:0", ...}
```

**Response:**
```http
HTTP/1.1 400 Bad Request

{
  "detail": "Invalid value: Authentication failed: Error code: 401 - {'error': {'message': 'Invalid API Key format: Must start with pre-defined prefix', ...}}"
}
```

---

### Subtest B: Empty API Key (Fallback to Config)

**Request:**
```http
POST /v1/chat/completions HTTP/1.1
x-llamastack-provider-data: {"aws_bedrock_api_key": ""}

{"model": "bedrock-inference/openai.gpt-oss-20b-1:0", ...}
```

**Response:**
```http
HTTP/1.1 200 OK

{
  "choices": [{
    "message": {"content": "...Hello! How can I assist you today?"}
  }]
}
```

 **Fell back to config key**

---

### Subtest C: Malformed Token

**Request:**
```http
POST /v1/chat/completions HTTP/1.1
x-llamastack-provider-data: {"aws_bedrock_api_key": "not-a-valid-bedrock-token-format"}

{"model": "bedrock-inference/openai.gpt-oss-20b-1:0", ...}
```

**Response:**
```http
HTTP/1.1 400 Bad Request

{
  "detail": "Invalid value: Authentication failed: Error code: 401 - {'error': {'message': 'Invalid API Key format: Must start with pre-defined prefix', ...}}"
}
```

src/llama_stack/distributions/ci-tests/run.yaml

@@ -46,6 +46,9 @@ providers:
       api_key: ${env.TOGETHER_API_KEY:=}
   - provider_id: bedrock
     provider_type: remote::bedrock
+    config:
+      api_key: ${env.AWS_BEDROCK_API_KEY:=}
+      region_name: ${env.AWS_DEFAULT_REGION:=us-east-2}
   - provider_id: ${env.NVIDIA_API_KEY:+nvidia}
     provider_type: remote::nvidia
     config: