Add support for Amazon Bedrock provider (#1185)

- follows existing patterns for AI SDK to provide Bedrock integration
- Uses Bedrock's API token feature for authentication which provides a
standard experience
- bedrock provided models match the Anthropic provided models (for now)


**Disclaimer**: The contributing docs are extremely sparse. I don't
actually know how to build this and get this running in Electron


## Testing

- AWS Bedrock provider is available for selection
<img width="994" height="496" alt="image"
src="https://github.com/user-attachments/assets/3cb21fed-9826-40e5-8019-b2b5df5e873b"
/>

- The provider settings also show the right models and offer the right
env variable to use
<img width="949" height="862" alt="image"
src="https://github.com/user-attachments/assets/8c23d5c8-d84d-4bf7-856a-8dc8d9d6c4b4"
/>


    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Adds AWS Bedrock as a provider so users can run Claude models via
Bedrock with API token authentication. The settings now list Bedrock
with supported models and a new env var.

- New Features
- New provider: bedrock using @ai-sdk/amazon-bedrock, wired into model
client and schemas.
- Models: Claude 4 Sonnet, Claude 3.7 Sonnet, Claude 3.5 Sonnet (Bedrock
model IDs).
- Settings: shows AWS Bedrock with correct models and env var
AWS_BEARER_TOKEN_BEDROCK.
  - Default region: us-east-1.

- Migration
  - Set AWS_BEARER_TOKEN_BEDROCK with your Bedrock API token.
  - Select AWS Bedrock in settings and pick a model.

<!-- End of auto-generated description by cubic. -->

Co-authored-by: Samrat Jha <samratj@amazon.com>
Co-authored-by: Will Chen <willchen90@gmail.com>

src/lib/schemas.ts

@@ -36,6 +36,7 @@ const providers = [
   "ollama",
   "lmstudio",
   "azure",
+  "bedrock",
 ] as const;
 
 export const cloudProviders = providers.filter(