Add failing tests for #451: default_language ignored in language detection

[Serhan-Asad]

Summary

Adds failing tests that detect the bug reported in #451 where the default_language setting in .pddrc is ignored during language detection.

Test Files

• Unit test: tests/test_construct_paths.py::test_default_language_fallback_from_pddrc
• E2E test: tests/test_e2e_issue_451_default_language.py (5 comprehensive tests)

What This PR Contains

• Failing unit test that reproduces the exact bug with isolated language detection logic
• Failing E2E test suite that verifies the bug at integration level through the full CLI path
• Tests are verified to fail on current code and will pass once the bug is fixed

Root Cause

Location: pdd/construct_paths.py:688 in _determine_language()

Issue: Key name mismatch where:

1. Configuration correctly stores value as command_options["default_language"]
2. Language detection checks command_options.get("language") (wrong key!)
3. No fallback logic exists to check default_language before raising error at line 762

Impact: Users configure default_language: python in .pddrc, but pdd generate test.prompt (without language suffix) fails with "Could not determine language from input files or options."

The Fix

Add 4 lines before line 762 in pdd/construct_paths.py:

# 5 - Check default_language from .pddrc as fallback
default_lang = command_options.get("default_language")
if default_lang:
    return default_lang.lower()

# 6 - If no language determined, raise error
raise ValueError("Could not determine language from input files or options.")

This establishes the correct priority order:

1. CLI --language flag (highest priority)
2. Code file extension (.py → python)
3. Prompt filename suffix (_python.prompt → python)
4. default_language from .pddrc (NEW FALLBACK)
5. Error if none found

Test Coverage

The test suite verifies:

• ✅ Main bug: default_language from .pddrc is used as fallback
• ✅ Priority: Filename suffix overrides config default
• ✅ Priority: CLI flag overrides config default
• ✅ Priority: File extension overrides config default
• ✅ Regression: Error still raised when NO language source exists
• ✅ Edge case: Case-insensitive handling

Next Steps

1. Implement the fix at pdd/construct_paths.py:762
2. Verify unit test passes: pytest tests/test_construct_paths.py::test_default_language_fallback_from_pddrc -v
3. Verify E2E tests pass: pytest tests/test_e2e_issue_451_default_language.py -v
4. Run full test suite: pytest tests/
5. Mark PR as ready for review

Fixes #451

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Generated by PDD agentic bug workflow (Steps 1-10)