Add failing tests for #412: incorrect sys.modules paths

[prompt-driven-github]

Summary

Adds failing tests that detect the bug reported in #412, where PDD generates tests with incorrect sys.modules paths that don't match actual import statements in source code.

Test Files

• Unit test: tests/test_e2e_issue_412_sys_modules_paths.py
• E2E test: tests/test_e2e_issue_412_cli_sys_modules_paths.py

What This PR Contains

• Failing unit test that reproduces the reported bug by validating that generated sys.modules keys match actual import paths from source code
• Failing E2E test that verifies the bug at integration level, exercising the full PDD CLI workflow with realistic project structures
• Tests are verified to fail on current code and will pass once the bug is fixed

Root Cause

PDD does not extract actual import statements from source code before generating tests. It only passes the filename stem (e.g., "firestore_client") as module_name to the LLM at pdd/cmd_test_main.py:193, forcing the LLM to guess which module paths need mocking in sys.modules. This results in inconsistent inference where sometimes sys.modules["config"] is generated (wrong) when the source uses from src.config import ..., and sometimes sys.modules["src.config"] is generated (correct).

Next Steps

1. Implement the fix at pdd/cmd_test_main.py to extract actual import paths using Python's ast module
2. Verify the unit test passes
3. Verify the E2E test passes
4. Run full test suite
5. Mark PR as ready for review

Fixes #412

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