fix(tests): Fix Docker build test after Poetry root migration

Updated test to copy pyproject.toml and poetry.lock from project root
instead of backend/ directory.

Changes:
- Added pyproject.toml and poetry.lock to root files_to_copy list
- Removed these files from backend_files list
- Added comment explaining Poetry root migration (Issue #501)

This fixes the Docker build failure:
  ERROR: failed to compute cache key: "/pyproject.toml": not found

Root cause: Makefile test was copying Poetry files from backend/ but
they've been moved to project root in the Poetry migration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: manavgup

tests/test_makefile_targets_direct.py

@@ -37,7 +37,10 @@ def setup_test_environment(self) -> None:
                 "docker-compose.dev.yml",
                 "docker-compose-infra.yml",
                 "env.example",
-                "env.dev.example"
+                "env.dev.example",
+                # Poetry files moved to root (Issue #501 - October 2025)
+                "pyproject.toml",
+                "poetry.lock"
             ]
 
             # Copy files
@@ -53,9 +56,9 @@ def setup_test_environment(self) -> None:
                 # Copy essential backend files and directories needed by Dockerfile
                 backend_dst.mkdir(parents=True, exist_ok=True)
 
-                # Copy root files
+                # Copy backend-specific files
                 backend_files = [
-                    "main.py", "healthcheck.py", "pyproject.toml", "poetry.lock"
+                    "main.py", "healthcheck.py"
                 ]
                 for file in backend_files:
                     src_file = backend_src / file