feat(datafusion): add catalog-level temporary table support

[JingsongLi]

Purpose

• Add register_temp_table/deregister_temp_table to PaimonCatalogProvider for session-scoped in-memory tables under {catalog}.__temp.{table_name}
• Refactor merge_into, delete, and update modules to use &SQLContext directly instead of &SessionContext + catalog_name, getting catalog via ctx.current_catalog_name() internally
• Introduce shared TempTableTracker in merge_into (pub(crate)) to replace per-module TempTableRegistry, delegates registration to SQLContext
• Update all internal execution functions to call ctx.ctx() when invoking DataFusion's SessionContext methods to avoid recursive SQL routing
• Add tests and documentation for temporary table usage

Brief change log

Tests

API and Format

Documentation