fix(workflow): preserve unresolved positional placeholders

[mikamikasuki]

Workflow execution crashes on bare positional placeholders such as {0} or {} because formatting receives only named context values. Preserve those strings through the existing unresolved-placeholder path while keeping named indexing and formatting errors visible. Input defaults retain their existing behavior. Regression tests run through both pytest and the file’s direct harness.

Fixes #154

Validation: uv run --frozen --with pytest --with pytest-asyncio python -m pytest --asyncio-mode=auto tests/test_workflow_execution.py -q and uv run --frozen --with pytest --with pytest-asyncio python tests/test_workflow_execution.py each pass 17 tests. Scoped ruff check and ruff format --check pass.

[cubic-dev-ai]

All reported issues were addressed across 2 files

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Fix all with cubic | Re-trigger cubic</sub>