Repair banana recipe test suite

[daxia778]

Summary

• replace the broken recipes.banana_pudding placeholder with a deterministic recipe model, validation helpers, markdown export, and safety manifest
• repair the banana recipe tests so pytest can collect and run the recipe suite without the previous import/name/syntax failures
• avoid the duplicate banana_recipes_test.py basename by moving the markdown coverage into a uniquely named test module

Closes #8

Validation

• python3 -m pytest src/test_banana_recipes_markdown.py src/test_banana_pudding_test.py src/tests/banana_recipes_test.py -q
• python3 -m py_compile src/recipes/banana_pudding.py src/test_banana_recipes_markdown.py src/test_banana_pudding_test.py src/tests/banana_recipes_test.py
• git diff --check -- src/recipes/banana_pudding.py src/test_banana_recipes_markdown.py src/test_banana_pudding_test.py src/tests/banana_recipes_test.py

Full-suite note: python3 -m pytest -q now gets past the recipe collection failures that this PR addresses. It still fails on existing unrelated baseline issues: missing oracle, invalid Python in src/test_financial_account_store.py, and missing mcp.