test: add generated BDD test runner

[nogates]

Summary

• add the generated portable test data, runner, and server
• wire the BDD suite to the generated runner when DD_USE_GENERATED_TESTS=true
• display === Using Generated Test Runner === once the generated runner is active

This is the functional first half of the transformer migration split. Formatter/pre-commit changes remain isolated on transformer-migration.

After this PR merges and generation reruns, #3872 will contain the remaining generated formatting/codegen differences.

[nogates]

Regeneration audit completed:

• this PR is one signed commit directly on SDK base 1ecd88d85c5ffef19517b9517cd1faaa9f7d876e
• feature synchronization and portable artifacts were regenerated from datadog-api-spec/master at e20c4541942fd4f7c8d54f1b4f12d3a68d4ff410
• generation used openapi-transformer at 74070c99
• output contains 1,692 request plans and 1,878 recordings, including both new monitor bundle_config scenarios
• no SDK cassette, Go module, or pre-commit files are changed

This replaces the earlier payload copied from the spec migration run, which was stale relative to current master.