refactor(core): use sqlalchemy orm and table reflection in sql resource store

[AlessandroPomponio]

Summary

This PR reworks the metastore's SQL layer to replace ad-hoc raw SQL string queries and manual table-existence bookkeeping with SQLAlchemy ORM constructs. The result is a cleaner, more maintainable, and more type-safe database layer that no longer depends on dialect-specific raw SQL for introspection.

High-level Changes

• SQLAlchemy ORM queries throughout: All raw sqlalchemy.text(...) queries in the resource store (filtering, joining, traversal, relationship lookups) have been replaced with ORM-style constructs using Table, select, and_, column operators, and explicit aliases.
• Schema reflection replaces manual table-existence checks: The _tables_exist_cache mechanism and the ensureExists parameter have been removed. Instead, create_sql_resource_store now returns a MetaData object that is cached via _reflected_metadata_cache, giving the store direct, typed references to its tables (_resources_table, _relationships_table) without extra round-trips.
• table_exists_query and check_table_exists removed: The dialect-specific raw SQL helpers for checking table existence have been deleted along with the now-unnecessary statements.py export surface.
• json_extract_field_as_string utility added: A new helper wraps the ->> JSON extraction operator in a cast(..., String) to prevent SQLAlchemy's JSON type processor from attempting to deserialise already-extracted scalar values, fixing a latent JSONDecodeError.
• Pickle/unpickle support updated: __getstate__/__setstate__ now include the table references so they are properly dropped and re-reflected across Ray serialisation boundaries.
• Tests updated: Existing tests are updated to match the new interface; new tests cover the resource store and updated SQL utility helpers.

Impact

These are internal implementation changes with no effect on the public ado API, CLI behaviour, or resource schemas. Performance is unchanged or slightly improved by eliminating redundant table-existence network queries. Any code that previously passed ensureExists to SQLResourceStore directly must be updated, but that parameter was not part of the public API.

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AI Disclosure: The code changes and this PR description were generated using IBM Bob and were reviewed manually.