plpgsql: report ambiguous column error for conflicting variable names

By default, when a PLpgSQL variable conflicts with a column name, the result is an ambiguous column error. It is also possible to change the `plpgsql.variable_conflict` setting in order to prefer either columns or variables in such cases. See the postgres docs here: https://www.postgresql.org/docs/current/plpgsql-implementation.html#PLPGSQL-VAR-SUBST

Currently, CRDB resolves ambiguous cases as column references rather than variable references.

Jira issue: CRDB-34151

Start with the PostgreSQL PL/pgSQL variable-conflict documentation linked in the issue, then locate CockroachDB’s existing PL/pgSQL name-resolution handling and regression tests. Confirm the current column-preference behavior, add coverage for an ambiguous variable and column reference, and ensure it reports the ambiguity as PostgreSQL does.

Written by the indexing model from the issue text.