Fix Dependency Output Warning Pollution

Tolerate non-JSON warnings in tofu/terraform output -json

Resolving dependency outputs no longer fails when the underlying tofu/terraform output -json invocation prints a deprecation warning to stdout alongside the JSON payload. Terraform 1.15.0 introduced a backend deprecation warning for the S3 dynamodb_table parameter that is emitted on stdout after the JSON object, which broke parsing with errors like invalid character 'W' after top-level value and the misleading downstream message There is no variable named "dependency".

Terragrunt now isolates the first JSON object in the captured stdout, so leading log lines (for example, the long-standing AWS Client Side Monitoring Enabling CSM line) and trailing warning blocks are both ignored when reading dependency outputs.

Resolves #6001. Thanks to @jpke for contributing this fix!