Clarify DuckLake metadata query intent

[bill-ph]

Summary

• Split DuckLakeTransaction metadata query entry points into Execute, SnapshotQuery, CurrentQuery, and RawQuery.
• Mirror the explicit read/write API split in DuckLakeMetadataManager and PostgresMetadataManager.
• Reclassify call sites so metadata DDL/DML uses Execute, snapshot-visible metadata reads use SnapshotQuery, current/latest metadata reads use CurrentQuery, and local DuckDB utility calls use RawQuery.

Why

This is Phase 3 of the metadata transaction-duration plan. It makes metadata call-site intent explicit before a later behavioral PR changes metadata transaction ownership. This PR does not shorten metadata transaction lifetime by itself; GetConnection still creates the metadata connection and begins its transaction.

Verification

• cmake --build build/release --target ducklake_extension
• ./build/release/test/unittest test/sql/stats/global_stats.test
• ./build/release/test/unittest test/sql/stats/global_stats_transactions.test
• ./build/release/test/unittest test/sql/data_inlining/inlined_data_multi_table_fresh_connection.test
• ./build/release/test/unittest test/sql/add_files/add_files_transaction_local.test
• ./build/release/test/unittest test/sql/transaction/basic_transaction.test
• ./build/release/test/unittest test/sql/table_changes/ducklake_table_deletions_inlined_flush.test
• ./build/release/test/unittest test/sql/deletion_inlining/test_deletion_inlining_transaction.test

Note: test/sql/metadata/postgres_metadata_passthrough.test was skipped locally because this build does not have postgres_scanner available.