Migrate YAML workflow Examples to DSL

[mathiusj]

This issue tracks adding VCR (Video Cassette Recorder) infrastructure to DSL functional tests, enabling HTTP request recording/playback for testing workflows that make API calls without requiring real credentials or incurring costs.

Context

Currently, DSL functional tests only cover workflows that don't make HTTP requests (like loops, conditionals, file operations). We need VCR support to test chat and agent workflows that interact with external APIs.

Implementation Tasks

✅ Core VCR Infrastructure

• Implement VCR for the DSL functional tests (refer to how @dersam set it up for the legacy functional tests in test/support/functional_test.rb, but leave that setup as-is)

🔄 Basic Chat Workflow Tests

• Add the dsl/simple_chat.rb example DSL workflow to the functional test suite
• Write some additional example workflows that use chat in more interesting, complex, real-world ways, with VCR to run them in tests

🔄 Agent Workflow Support

• Figure out a good way to mock agent calls in functional tests
• Add a functional test case for dsl/simple_agent.rb using that mocking setup
• Write some additional example workflows that use agent (perhaps in combination with chat) and hook them up as functional tests

Success Criteria

• DSL functional tests can record/playback HTTP requests for chat workflows
• Agent workflows can be tested with appropriate mocking
• New example workflows demonstrate real-world usage patterns
• All tests run reliably in CI without external dependencies

Related Files

• test/dsl/support/functional_test.rb - DSL VCR infrastructure
• test/dsl/functional/roast_dsl_examples_test.rb - Test cases
• dsl/simple_chat.rb - Basic chat example
• dsl/simple_agent.rb - Basic agent example