feat(bedrock): prompt for region on bedrock provider

[OpeOginni]

Issue for this PR

Closes #28834

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• Adds prompt for AWS region when trying to connect to the bedrock provider::This particularrly helps Desktop users who don't make use of the terminal to set ENV variables, they can simply set the region on the dialog.
• Improve docs showing that the Region ENV is very important to set if using credentials.

The Flow:

• If AWS_REGION env exists: no region prompt.
• If AWS_REGION env is missing: prompt for required Bedrock region.
• Region from the prompt is stored as auth metadata.

How did you verify your code works?

• Tests and typechecks work
• Tried the opencode auth login fro amazon bedrock, and also tried to connect to the provider on desktop, the region prompt showed up.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR