[yeongseon]

Summary

• allow the public generate_openapi_spec API to scan a supplied FunctionApp before compilation
• document real binding routes, methods, route parameters, and custom prefixes for either decorator order
• preserve function-name fallback behavior when no explicit route exists
• clarify the required app scan for innermost @openapi usage

Verification

• uv run pytest (1024 passed, 29 skipped; 95.93% coverage)
• uv run ruff check src tests
• uv run ruff format --check src tests
• uv run mypy src tests
• uv run python -m build

Fixes #735