format_plpgsql drops block labels, compiler directives and cursor queries

[gmr]

format_plpgsql() has the silent-drop defect that #58 fixed for SQL. The guard added in #59 never exercises it, since CREATE FUNCTION re-lays out a body rather than formatting it. Run over the 41 PL/pgSQL bodies in the PostgreSQL 19 documentation corpus, 5 lose content in every style:

Dropped	Example
Block labels	<<outerblock>> DECLARE ... BEGIN ... END loses <<outerblock>>, so EXIT outerblock / outerblock.quantity name a block that no longer exists
Compiler directives	#variable_conflict use_variable, #print_strict_params on
A bound cursor's query	referrer_keys CURSOR IS SELECT * FROM cs_referrer_keys ORDER BY try_order; loses the whole query

One further body is not a fixed point (a second pass changes it).

Separately, one body fails to parse: OPEN $1 FOR SELECT * FROM table_1; (a positional parameter as the cursor variable). That is valid PL/pgSQL rejected by the tree-sitter-postgres grammar, so it is an upstream grammar gap rather than a libpgfmt bug.

Fix

• Render the label, directive and cursor-query nodes in the PL/pgSQL formatter.
• Extend tests/token_loss_test.rs to run format_plpgsql() over the corpus's PL/pgSQL bodies with the same three checks (no token loss, output re-parses, fixed point), so this class is covered.