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

[plusplusjiajia]

Purpose

Support VERSION AS OF <snapshot_id> for registered external table engines through SQLContext and DataFusion SessionContext.

SQLContext resolves snapshots even when the current provider cannot be loaded. Raw SessionContext preloads the current provider and propagates its errors.

Follow-up to #1100.

Brief change log

• Resolve external snapshot queries with the corresponding historical schema.
• Preserve resolver errors from direct DataFusion table lookups.
• Preserve read-only enforcement and native Paimon time-travel behavior.
• Add regression tests and update the DataFusion README.

Tests

• 531 DataFusion unit and integration tests passed.
• Clippy passed with warnings denied.
• Formatting and diff checks passed.