feat!: change bedrock bearer token env variable to match AWS docs & boto3 convention (#4152)

Rename `AWS_BEDROCK_API_KEY` to `AWS_BEARER_TOKEN_BEDROCK` to align with
the naming convention used in AWS Bedrock documentation and the AWS web
console UI. This reduces confusion when developers compare LLS docs with
AWS docs.

Closes #4147

docs/docs/providers/inference/remote_bedrock.mdx

@@ -22,6 +22,6 @@ AWS Bedrock inference provider using OpenAI compatible endpoint.
 ## Sample Configuration
 
 ```yaml
-api_key: ${env.AWS_BEDROCK_API_KEY:=}
+api_key: ${env.AWS_BEARER_TOKEN_BEDROCK:=}
 region_name: ${env.AWS_DEFAULT_REGION:=us-east-2}
 ```