feat(postgres): add identity/schema niladic functions to bare_functions

[skyf0l]

Summary

PostgreSQL allows a set of SQL-standard niladic functions to be called without parentheses — current_schema, current_user, current_catalog, current_role, session_user, system_user, and user. sqruff's postgres dialect only registered the date/time niladic functions (current_timestamp, current_time, current_date, localtime, localtimestamp) in its bare_functions set, so the identity/schema ones were not recognised as functions in general expression positions.

As a result, perfectly valid PostgreSQL was flagged as an unparsable section whenever one of these appeared as a function argument or in a multi-item select list:

-- flagged "Unparsable section" before this change
SELECT some_function(current_schema, 'x');
SELECT current_user, session_user;

while the parenthesised form parsed fine:

SELECT current_schema();   -- ok (parsed as an ordinary function call)

This change adds the missing niladic functions to the postgres bare_functions set so they parse correctly everywhere.

Motivation / reference

The fix aligns the postgres dialect with SQLFluff, which lists all of these in its postgres bare_functions set:

• SQLFluff dialect_postgres.py — bare_functions update: src/sqlfluff/dialects/dialect_postgres.py

PostgreSQL documentation confirms these have special syntactic status and are spelled without trailing parentheses:

current_catalog, current_role, current_schema, current_user, session_user, and user have special syntactic status in SQL: they must be called without trailing parentheses. (In PostgreSQL, parentheses can optionally be used with current_schema, but not with the others.)

— PostgreSQL: System Information Functions

Changes

• crates/lib-dialects/src/postgres.rs — extend the postgres bare_functions set with CURRENT_CATALOG, CURRENT_ROLE, CURRENT_SCHEMA, CURRENT_USER, SESSION_USER, SYSTEM_USER, and USER.
• crates/lib-dialects/test/fixtures/dialects/postgres/sqlfluff/bare_functions.sql — extend the fixture to cover the newly supported functions (mirrors the equivalent SQLFluff fixture).
• .../postgres/sqlfluff/bare_functions.yml — regenerated expected parse tree; the new columns now parse as bare_function.
• .../postgres/sqlfluff/alter_policy.yml and .../postgres/sqlfluff/create_policy.yml — regenerated. These existing fixtures use current_user inside policy USING (...) clauses; it was previously parsed as column_reference → naked_identifier and is now correctly classified as bare_function. No SQL fixture input changed — only the expected trees, reflecting the improved parse.

Testing

• Regenerated fixtures with env UPDATE_EXPECT=1 cargo test -p sqruff-lib-dialects --test dialects.
• Full dialect parse suite passes (cargo test -p sqruff-lib-dialects --test dialects), including all postgres fixtures — no unparsable segments and no regressions (notably, adding user did not break FROM user / user.col parses across the existing fixture corpus).
• Verified end-to-end that previously-unparsable expressions such as SELECT f(current_schema, ...) and SELECT current_user, session_user now lint without any unparsable-section findings.