Fix setup round trips for dotted namespace levels

[mattfaltyn]

Summary

polaris setup export flattened namespace paths with ".".join(...), while
setup apply reconstructed them with .split("."). That made a literal namespace level
such as ["finance.us"] indistinguishable from the nested namespace
["finance", "us"], affecting namespaces, policies, and namespace-scoped grants.

This change:

• exports namespace paths as YAML lists of levels everywhere setup state refers to a namespace;
• preserves tuple/list namespace identities internally while creating parents, policies, and grants;
• continues accepting legacy dot-delimited setup configurations;
• adds round-trip regression coverage for the colliding namespace, policy, and grant cases; and
• updates the changelog, setup guide, and reference configuration, including the compatibility note.

Fixes #5291

Validation

• make client-regenerate
• make client-unit-test — 187 passed
• make client-lint
• make client-license-check
• make client-build — sdist and wheel built successfully
• JAVA_HOME=/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home ./gradlew format compileAll
• JAVA_HOME=/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home DOCKER_HOST=unix:///Users/mattfaltyn/.colima/default/docker.sock TESTCONTAINERS_DOCKER_SOCKET_OVERRIDE=/var/run/docker.sock TESTCONTAINERS_HOST_OVERRIDE=127.0.0.1 TESTCONTAINERS_RYUK_DISABLED=true ./gradlew check
• git diff --check

Compatibility

Existing dot-delimited setup files remain accepted. Because the new export schema uses lists for
namespace paths, older CLI versions cannot apply newly exported files; this is documented in the
changelog.

Checklist

• 🛡️ No security issue is disclosed
• 🔗 The rationale is explained and the related issue is linked: Fixes Python CLI setup round trip collapses dotted namespace levels #5291
• 🧪 Tests cover namespace, policy, and namespace-grant round trips plus legacy input compatibility
• 💡 No comments were added because the representation-preserving logic is direct and localized
• 🧾 Updated CHANGELOG.md
• 📚 Updated site/content/in-dev/unreleased and the reference setup configuration

AI assistance

This change was prepared with significant AI assistance. I reviewed the implementation end to end
and remain responsible for the contribution.