Add custom region input support for AWS Bedrock provider

[richardwhiteii]

Feature Request: Custom Region Input for AWS Bedrock

Problem Statement:
When AWS launches new regions, users must wait for the codebase to be updated before they can access Bedrock services in those regions. This creates delays in adopting newly available AWS regions.

Proposed Solution:
Add a "Custom region..." option to the AWS Region dropdown that allows users to manually enter region identifiers for newly launched regions.

Detailed Description:

• Add "Custom region..." option at the end of the BEDROCK_REGIONS dropdown
• When selected, show a text input field for manual region entry
• Preserve existing region validation logic
• Handle state management for switching between standard and custom regions
• Maintain backward compatibility with existing configurations

Use Case:

• A new AWS region (e.g., us-west-3) becomes available with Bedrock support
• Users can immediately access it by entering the region manually
• No need to wait for a codebase update to add the new region to the hardcoded list

Implementation Notes:

• Similar pattern to existing Custom ARN functionality
• Maintains existing validation and error handling
• Non-breaking change that enhances existing functionality

Alignment with Roadmap:

• Reliability First: Expands robust support for AI providers by allowing immediate access to new regions
• Enhanced User Experience: Removes friction points by eliminating waiting periods for new region support

This enhancement would improve the user experience by providing immediate access to newly launched AWS regions without requiring code updates.

[roomote]

I am working on implementing the custom region input support for AWS Bedrock provider. This will add a "Custom region..." option to the AWS Region dropdown that allows users to manually enter region identifiers for newly launched regions. I will explore the codebase to understand the current implementation and provide a solution that maintains backward compatibility.