[codex] Avoid explicit metadata transactions for snapshot reads

[bill-ph]

Summary

Phase 4 of the metadata snapshot-read plan: normal snapshot-safe reads now avoid DuckLake's explicit long-lived metadata transaction path.

This keeps explicit DuckLake metadata transactions for write/current/maintenance paths while routing normal SnapshotQuery reads through a no-explicit-transaction metadata connection. Postgres-backed snapshot reads still go through postgres_query, so postgres-scanner may continue creating its own implicit read transaction underneath; that remains Phase 5 scope.

Changes

• Split DuckLake metadata connection creation so SnapshotQuery can run without BeginTransaction() while Execute, CurrentQuery, and RawQuery keep the explicit transaction path.
• Added SnapshotQueryInTransaction for maintenance/commit paths that still require explicit metadata transaction behavior.
• Threaded the explicit transaction intent through SQLite/default and Postgres metadata managers.
• Extended DuckLake metadata logging with metadata_api and ducklake_explicit_metadata_transaction so tests can assert the boundary directly.
• Added focused SQLLogic tests for normal snapshot reads, inlined snapshot reads, Postgres snapshot reads, pinned snapshot reads after newer commits, and maintenance preservation across flush/rewrite/cleanup.

Validation

• git diff --check
• python3 scripts/check_metadata_query_intent.py
• cmake --build build/release --target unittest -j 8
• cmake --build build/release --target duckdb -j 8
• cmake --build build/release --target ducklake_loadable_extension -j 8
• ./build/release/test/unittest --test-dir ./ test/sql/metadata/snapshot_reads_no_ducklake_tx.test
• ./build/release/test/unittest --test-dir ./ test/sql/metadata/inlined_snapshot_reads_no_ducklake_tx.test
• ./build/release/test/unittest --test-dir ./ test/sql/metadata/maintenance_snapshot_reads_keep_ducklake_tx.test
• ./build/release/test/unittest --test-dir ./ test/sql/metadata/snapshot_reads_stay_pinned_after_commits.test
• ./build/release/test/unittest --test-dir ./ test/sql/general/ducklake_metadata_logging.test
• PGHOST=127.0.0.1 PGUSER=postgres DUCKLAKE_CI=1 ./build/release/test/unittest --test-config test/configs/postgres.json --test-dir ./ test/sql/metadata/postgres_snapshot_reads_no_ducklake_tx.test