perf: cache site.getsitepackages() result

[KRRT7]

Summary

Cache site.getsitepackages() with @lru_cache(maxsize=1) to avoid repeated path resolution on every call to path_belongs_to_site_packages().

Part of #2132 — targeted E2E pipeline performance improvements.

Problem

site.getsitepackages() calls os.path.join + path resolution for every site-packages directory on every invocation. path_belongs_to_site_packages() is called hundreds of times per optimization run. Site-packages paths never change during a process lifetime — resolving them repeatedly is pure waste.

Solution

Extract _get_site_packages_paths() cached helper with @lru_cache(maxsize=1). First call resolves paths, all subsequent calls return the cached tuple.

Changes

• codeflash/code_utils/code_utils.py: Extract cached helper
• tests/test_code_utils.py: 4 new tests covering caching behavior, true/false paths, and symlinks

Test plan

• uv run pytest tests/test_code_utils.py -k site_packages -v
• Tests use cache_clear() to avoid cross-test contamination