feat(datafusion): support REST Catalog CREATE VIEW

[JingsongLi]

Summary

Add end-to-end persistent CREATE VIEW support for Paimon REST Catalogs through SQLContext. The defining query is planned in the view's owning namespace, its schema is inferred by DataFusion, and the Java-compatible view request is persisted through the catalog API.

Changes

• add CreateViewRequest, ViewSchema::new, Catalog::create_view, and ViewAlreadyExist
• implement REST create-view transport, HTTP 409 mapping, IF NOT EXISTS, and HTTP 501 fallback
• support CREATE VIEW [IF NOT EXISTS] name [(columns)] AS query in SQLContext
• infer field types/nullability, preserve the canonical DataFusion query, and bind bare relations/functions to the target catalog and database
• validate explicit column lists and reject unsupported persistent-view modifiers instead of silently ignoring them
• document the supported syntax and current limitations in the DataFusion README and SQL guide

Testing

• cargo test -p paimon --test rest_object_models_test --test rest_api_test --test rest_catalog_test
• cargo test -p paimon-datafusion --lib sql_context::tests -- --test-threads=1
• cargo test -p paimon-datafusion --lib persistent_rest_catalog_view_ -- --test-threads=1
• cargo clippy -p paimon -p paimon-datafusion --lib --tests -- -D warnings
• cargo fmt --all -- --check
• mkdocs build --strict

Notes

• Persistent CREATE VIEW is implemented by REST Catalog; other catalog implementations keep the default Unsupported behavior.
• CREATE OR REPLACE VIEW, materialized/secure views, comments/options, persistent ALTER VIEW / DROP VIEW, and persistent function DDL remain out of scope.

[leaves12138]

Thanks for adding persistent REST view creation. The main flow and the existing targeted tests look good, but I found two correctness issues that should be addressed before merge. I reproduced both with focused regression cases locally.

[leaves12138]

Re-reviewed the latest changes. Both previous issues are fixed correctly: COPY GRANTS is explicitly rejected with regression coverage, and CREATE VIEW now maps a missing parent database to DatabaseNotExist. The focused DataFusion and REST Catalog tests, formatting check, and diff check all pass locally. LGTM.