Allow DataSourceConfig to represent a plain database table

[pankajastro]

DataSourceConfig always inferred storage_type from uri, which defaults to empty. LLMSchemaCompareOperator builds a DataSourceConfig with only conn_id and table_name (no uri, no format) to introspect a plain database connection via DbApiHook, and that construction failed.

Reproduction (local airflow dags test run):

• Before the fix task plain_db_table would fail

@task
def plain_db_table():
    config = DataSourceConfig(conn_id="postgres_default", table_name="my_table")

• After the fix: the same task succeeds, storage_type stays None.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Sonnet 5)

Generated-by: Claude Code (Sonnet 5) following the guidelines

[kaxil]

Ran this end to end against a real Postgres backend in breeze: two real tables, a real Airflow connection, and each consumer of a plain-database DataSourceConfig.

The feature itself works. LLMSchemaCompareOperator now introspects a plain table through DbApiHook and returns real schema text:

Source: pg_e2e (postgresql)
Table: e2e_orders_v1
Columns: order_id INTEGER, customer VARCHAR(64), amount NUMERIC(10, 2), created_at TIMESTAMP
Index: e2e_orders_v1_customer_idx (customer)

That path was unreachable before, since the config could not be constructed, so the DbApiHook branch in _introspect_datasource_schema was dead code. Worth correcting in the description though: the operator does not build the config, the caller passes it in data_sources, and nothing in the repo constructs a plain-database one yet.

Where it needs another look is table_name validation. Same script, before and after moving the check above the new early return:

case	before	after
plain DB, valid table_name	OK, storage_type=None	OK, storage_type=None
plain DB, blank table_name	accepted	raises
plain DB, whitespace table_name	accepted	raises
explicit storage_type=S3, blank table_name, no uri	accepted	raises
uri set, blank table_name	raises	raises

Row three is a behaviour change against current main, which raises there today. Row two costs a worse error at task runtime: hook.get_table_schema("") surfaces a bare NoSuchTableError with an empty message, where the check would have said Table name must be provided at construction. I left a committable suggestion on the diff for it.

With the check hoisted, the full common.sql and common.ai unit suites pass (2084 tests), and both existing assertions on that message, in test_config.py and test_format_handlers.py, keep working.

On docs: llm_schema_compare.rst still scopes data_sources to object-storage and catalog-managed sources, so the new shape has no example anywhere. I have a short section written that frames it as the way to compare differently named tables, since db_conn_ids and table_names form a cross-product and cannot pair orders with orders_v2. Happy to hand it over for this PR or push it as a follow-up, whichever you prefer.

Separately, and for a follow-up rather than this PR: three of the four register_datasource callers do not gate on connection kind, so a plain-database config reaching LLMSQLOperator, DataFusionToolset or AnalyticsOperator fails with ValueError: Unknown connection type postgres from _get_credentials. Only LLMSchemaCompareOperator checks _is_dbapi_connection first.

[pankajastro]

Pushed an update: hoisted the table_name check (also catches the iceberg-blank-table_name case), added a check requiring uri when storage_type is set, and updated the docs/example DAG for the plain-DB shape. Left the register_datasource guard for a separate follow-up PR. Thanks for the thorough verification!

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Drafted-by: Claude Code (Sonnet 5); reviewed by @pankajastro before posting

[kaxil]

Verified the last push independently rather than off the diff alone. Extracted config.py at apache/main and at this head by SHA and exec'd both side by side: the storage_type=LOCAL/S3-with-no-uri case that regressed in the previous push now matches main again (ok, not a raise), and the format-with-no-uri case gets the new, reachable message instead of the old Unsupported storage type for URI: on an empty string. Every other delta from main is an intended part of this PR: the bug it fixes, and the justified iceberg blank-table_name tightening backed by its own test.

Full common.sql + common.ai unit suites pass (2086 passed, 7 skipped, mysql-only), and mypy is clean on config.py and the example DAG. I could not get a clean build-docs common.ai in my session, but that's an environment artifact, not this PR's: this branch's merge-base predates main's DuckDB provider, so the container's provider-discovery metadata references a module this checkout doesn't have. Round 2's docs build was clean on nearly the same RST content, and this round's diff to it is wording only.

Also checked #73287 directly: it guards DataFusionEngine.register_datasource itself, which covers the three previously-ungated call sites at their one choke point rather than patching each individually, with a test for the plain-DB case.