docs: add CHANGELOG entry for v0.5.2

[RonaldHensbergen]

Adds the dated ## [0.5.2] - 2026-08-25 section that #515 (chore(release): bump version to 0.5.2) intentionally left for a follow-up, per that PR's description.

Documents the two feature/fix PRs merged into 0.5.2:

• fix: prefer .cds marker for project root, add full-stack install smoke test #512 - .cds marker preferred for project root resolution (find_project_root/resolve_project_root), plus a full-stack install smoke test in build-python-package.yml.
• Make cds get manifest parsing fail-safe and observable #495 - cds get manifest parsing is now fail-safe: malformed/unreadable .cds/get-manifest.json is backed up and reported instead of silently discarded.

[SemTiOne]

Nice!