fix: PDD fix for #687

[prompt-driven-github]

Summary

Adds failing tests that detect the bug reported in #687 — code_generator_main.py never injects example_output_path from resolved_config into template env_vars, causing the LLM to emit raw source code paths instead of _example.py file paths.

Test Files

• Unit tests: tests/test_code_generator_main.py (5 new tests added)
• E2E test: tests/test_e2e_issue_687_example_output_path.py

What This PR Contains

• 5 failing unit tests that reproduce the reported bug (3 fail detecting the bug, 2 pass covering edge cases)
• 1 failing E2E test that verifies the bug at integration level through the real code_generator_main → construct_paths → _expand_vars pipeline
• Tests are verified to fail on current code and will pass once the bug is fixed

Root Cause

code_generator_main.py resolves example_output_path from the config via construct_paths() but never injects it into env_vars for template variable substitution. When .pddrc is missing, the LLM has no way to discover example file paths and falls back to emitting raw source code paths like src/core/storage_layer.py.

Next Steps

1. Implement the fix in code_generator_main.py — inject EXAMPLE_OUTPUT_PATH into env_vars
2. Verify the unit tests pass
3. Verify the E2E test passes
4. Run full test suite
5. Mark PR as ready for review

Fixes #687

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