fix(datafusion): keep metadata queries available without table engines

[shyjsarah]

Purpose

After external table routing was made fail-closed, DataFusion metadata queries such as information_schema.columns could fail if a catalog contained any external table whose engine was not registered. This also surfaced through statements that inspect catalog metadata.

Changes

• preserve catalog field metadata when classifying external tables
• return a metadata-only provider when the declared table engine is unavailable
• expose the real table schema to DataFusion metadata queries
• make table_exist() agree with the metadata-only provider for catalog tables without a registered engine
• bypass read-specific session option validation only for metadata-only loading; registered-engine reads still reject unsupported time travel, incremental scan, and query-auth options
• keep scans and writes fail-closed with the existing no table engine is registered error
• cover SHOW TABLES, unrelated and external information_schema.columns queries, direct reads, destructive statements, and table_exist

Behavior

Operation	Unregistered external table
SHOW TABLES	succeeds and lists the table
information_schema.columns	succeeds using catalog metadata
SessionContext::table_exist	returns true for the catalog table
direct SELECT	fails when the scan is planned
writes	fail closed

Tests

• cargo fmt --all -- --check
• cargo +1.98.0 clippy --locked -p paimon -p paimon-datafusion --all-targets --features fulltext,vortex -- -D warnings
• cargo test -p paimon-datafusion --test table_type_routing

[JingsongLi]

• When no engine is present, table() returns a metadata-only provider, yet table_exist() returns false, violating the existence semantics of the DataFusion SchemaProvider.
• Consequently, the same external table can be queried via information_schema but appears non-existent when checked via SessionContext::table_exist.

[shyjsarah]

Addressed in d3200f4: when no engine is registered, table_exist() now returns true to match the metadata-only provider returned by table(). The regression also verifies metadata loading under a session time-travel option.

[jerry-024]

+1