fix(druid): add JSON_VALUE RETURNING support

[sitelight]

Description

Adds JSON_VALUE to Druid's FUNCTION_PARSERS so the RETURNING <type> clause is parsed instead of falling through to the generic function-call parser, which can't handle it.

The base Parser._parse_json_value already handles RETURNING (and ON EMPTY / ON ERROR) correctly — it just wasn't routed for the Druid dialect. MySQL, Trino, and Exasol register JSON_VALUE the same way.

Before

>>> import sqlglot
>>> sqlglot.parse_one("SELECT JSON_VALUE(x, '$.a' RETURNING BIGINT) FROM t", dialect="druid")
sqlglot.errors.ParseError: Expecting ). Line 1, Col: 36.

After

>>> sqlglot.parse_one("SELECT JSON_VALUE(x, '$.a' RETURNING BIGINT) FROM t", dialect="druid").sql(dialect="druid")
"SELECT JSON_VALUE(x, '$.a' RETURNING BIGINT) FROM t"

Related

Surfaced via apache/superset#36954 — Superset users on Druid datasets currently can't run any query using json_value(..., '$.x' RETURNING <type>) because the SQL fails sanitize/format before being sent to the engine.

Tests

Added test_json_value to tests/dialects/test_druid.py covering plain JSON_VALUE, RETURNING DOUBLE, and the exact failure case from the linked issue. Full suite (python -m unittest) green locally.

[georgesittas]

Thanks!