fix(sqlalchemy): do not emit CREATE SCHEMA on Oracle (#3939)

[DRACULA1729]

Description

Loading to Oracle through the sqlalchemy destination fails on the first run with ORA-02420: missing schema authorization clause. dlt runs CREATE SCHEMA "<dataset>" when it cannot find the target dataset, and Oracle rejects that. In Oracle a schema is a database user, so there is no bare CREATE SCHEMA (it needs CREATE SCHEMA AUTHORIZATION <user> ...).

This adds dataset-lifecycle extension points to DialectCapabilities and routes the SQL client through them. The base implementations keep the old behavior for every other dialect, so nothing changes for Postgres/MySQL/etc.

OracleDialectCapabilities overrides them:

• dataset_exists matches case-insensitively, since Oracle folds unquoted identifiers to upper case. Loading into an existing schema is now detected and no creation is attempted.
• create_dataset no longer emits CREATE SCHEMA. If the schema is missing it raises a terminal error explaining that the schema (and the <dataset>_staging schema, for merge/replace) has to be created up front as a user with grants, instead of surfacing the cryptic ORA-02420.
• drop_dataset drops the tables inside the schema instead of DROP SCHEMA, which on Oracle would require DROP USER (a DBA privilege).

Related Issues

• Fixes Oracle 12c ORA-02420: missing schema authorization clause #3939

Additional Context

• Follows the maintainer guidance in Pipeline state tables are not created when using Oracle via SQLAlchemy as a destination #3141: staging features are not hard-restricted; if the user pre-creates the <dataset>_staging schema, merge/replace still work.
• Oracle is not in the sqlalchemy-destination CI matrix, so the new tests are unit-level (mock-based) in tests/load/sqlalchemy/test_sqlalchemy_dialect.py and run without a live Oracle.
• Docs updated: the Oracle limitations section and the dialect-capabilities extension-points table in sqlalchemy.md.
• Ran locally: the new dialect tests pass; ruff, mypy, flake8 and black are clean on the changed files.

[DRACULA1729]

This is ready for review. The fork-gated workflows (the ones that need repo secrets) are stuck in the "requires reviewer approval" state, so they need a maintainer to approve the run before they execute.

@ivasio you dug into this exact Oracle case in #3141, so it's probably familiar territory. @rudolfix it extends the DialectCapabilities system from #3600 with dataset-lifecycle hooks. Happy to rebase or adjust anything.