feat(datafusion): support timestamp time travel for external engines

[plusplusjiajia]

Purpose

Support TIMESTAMP AS OF '<yyyy-MM-dd HH:mm:ss>' for registered external table engines through SQLContext and DataFusion SessionContext:

SELECT * FROM lake.db.events TIMESTAMP AS OF '2026-10-01 00:00:00';

The timestamp is parsed as for Paimon tables (local time zone, epoch milliseconds) and passed to the engine, which picks the snapshot.

Follow-up to #1113.

Brief change log

• Add TableEngineResolver::resolve_table_at_timestamp. The default implementation rejects timestamp time travel, so existing resolvers keep failing closed.
• Route TIMESTAMP AS OF on external data tables through the same read-only wrapper and session option checks as VERSION AS OF.
• Keep the existing boundaries: SQLContext resolves without loading the current provider; raw SessionContext preloads it and propagates its errors; branches, metadata tables and FOR SYSTEM_TIME AS OF remain rejected.
• Update the DataFusion README.

Tests

• Timestamps at and between engine snapshot commit times reach the resolver as exact epoch milliseconds and return that snapshot's data and schema.
• SQLContext reads a timestamp snapshot when the current provider cannot be resolved; raw SessionContext propagates the current resolution error.
• Missing snapshots, backend failures and missing tables are reported without falling back to current data; resolvers without timestamp support and unregistered engines are rejected.
• Timestamp providers reject writes; session scan options and query-auth are checked before the engine is called.
• Paimon tables keep Paimon's own timestamp resolution, including when joined with an external timestamp snapshot.