feat: bedrock streaming optimisations (#602)

## Summary

Implements proper AWS EventStream handling for Bedrock streaming responses, replacing the manual buffer parsing with the official AWS SDK eventstream decoder.

Solves #597

## Changes

- Added `aws/aws-sdk-go-v2/aws/protocol/eventstream` dependency to properly decode AWS EventStream format
- Refactored `ChatCompletionStream` method in the Bedrock provider to use the official AWS EventStream decoder
- Moved the AWS request signing function to improve code organization
- Added proper handling of tool calls in streaming responses
- Fixed empty braces handling in tool call arguments to avoid duplication
- Improved error handling and response processing for streaming responses

## Type of change

- [x] Bug fix
- [x] Feature
- [x] Refactor
- [ ] Documentation
- [ ] Chore/CI

## Affected areas

- [x] Core (Go)
- [ ] Transports (HTTP)
- [x] Providers/Integrations
- [ ] Plugins
- [ ] UI (Next.js)
- [ ] Docs

## How to test

Test Bedrock streaming with various models, especially those that support tool calls:

```sh
# Core/Transports
go version
go test ./core/providers/bedrock_test.go

# Test streaming with Claude models
curl -X POST http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "anthropic.claude-3-sonnet-20240229-v1:0",
    "messages": [{"role": "user", "content": "Tell me a short story"}],
    "stream": true
  }'

# Test streaming with tool calls
curl -X POST http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "anthropic.claude-3-sonnet-20240229-v1:0",
    "messages": [{"role": "user", "content": "What is the weather in New York?"}],
    "stream": true,
    "tools": [{"type": "function", "function": {"name": "get_weather", "description": "Get the weather in a location", "parameters": {"type": "object", "properties": {"location": {"type": "string"}}, "required": ["location"]}}}]
  }'
```

## Breaking changes

- [x] No
- [ ] Yes

## Related issues

Improves Bedrock streaming reliability and performance, especially for tool calls.

## Security considerations

No security implications as this change only affects how we parse AWS EventStream responses.

## Checklist

- [x] I added/updated tests where appropriate
- [x] I verified builds succeed (Go and UI)
- [x] I verified the CI pipeline passes locally if applicable

Author: akshaydeo

core/go.mod

@@ -6,6 +6,7 @@ toolchain go1.24.3
 
 require (
 	github.com/aws/aws-sdk-go-v2 v1.38.0
+	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.6.10
 	github.com/aws/aws-sdk-go-v2/config v1.31.0
 	github.com/bytedance/sonic v1.14.0
 	github.com/google/uuid v1.6.0

core/go.sum

@@ -4,6 +4,8 @@ github.com/andybalholm/brotli v1.2.0 h1:ukwgCxwYrmACq68yiUqwIWnGY0cTPox/M94sVwTo
 github.com/andybalholm/brotli v1.2.0/go.mod h1:rzTDkvFWvIrjDXZHkuS16NPggd91W3kUSvPlQ1pLaKY=
 github.com/aws/aws-sdk-go-v2 v1.38.0 h1:UCRQ5mlqcFk9HJDIqENSLR3wiG1VTWlyUfLDEvY7RxU=
 github.com/aws/aws-sdk-go-v2 v1.38.0/go.mod h1:9Q0OoGQoboYIAJyslFyF1f5K1Ryddop8gqMhWx/n4Wg=
+github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.6.10 h1:zAybnyUQXIZ5mok5Jqwlf58/TFE7uvd3IAsa1aF9cXs=
+github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.6.10/go.mod h1:qqvMj6gHLR/EXWZw4ZbqlPbQUyenf4h82UQUlKc+l14=
 github.com/aws/aws-sdk-go-v2/config v1.31.0 h1:9yH0xiY5fUnVNLRWO0AtayqwU1ndriZdN78LlhruJR4=
 github.com/aws/aws-sdk-go-v2/config v1.31.0/go.mod h1:VeV3K72nXnhbe4EuxxhzsDc/ByrCSlZwUnWH52Nde/I=
 github.com/aws/aws-sdk-go-v2/credentials v1.18.4 h1:IPd0Algf1b+Qy9BcDp0sCUcIWdCQPSzDoMK3a8pcbUM=