Clarify DuckLake metadata query intent

[bill-ph]

Summary

• split DuckLake metadata execution into Execute, SnapshotQuery, CurrentQuery, and RawQuery
• classify existing metadata call sites without changing transaction behavior
• add CI-backed metadata Query intent audit

Testing

• cmake --build build/release --target ducklake_extension -j 8
• ./build/release/test/unittest test/sql/data_inlining/data_inlining_time_travel_reattach.test
• ./build/release/test/unittest test/sql/data_inlining/data_inlining_flush_schema.test
• ./build/release/test/unittest test/sql/metadata/ducklake_settings.test
• python3 scripts/check_metadata_query_intent.py
• PYTHONPYCACHEPREFIX=/private/tmp/ducklake-pycache python3 -m py_compile scripts/check_metadata_query_intent.py
• git diff --check

Notes: sqlite/postgres-specific metadata tests were invoked earlier but skipped because the local build is missing sqlite_scanner/postgres_scanner requirements.

[bill-ph]

Closing in favor of repo-owned PR #19 from PostHog:codex/metadata-api-semantics.