refactor: fix prepared_statement.c naming to follow HACKING.md conventions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: suketa

ext/duckdb/duckdb.c

@@ -47,7 +47,7 @@ Init_duckdb_native(void) {
     rbduckdb_init_duckdb_result();
     rbduckdb_init_column();
     rbduckdb_init_logical_type();
-    rbduckdb_init_duckdb_prepared_statement();
+    rbduckdb_init_prepared_statement();
     rbduckdb_init_pending_result();
     rbduckdb_init_duckdb_blob();
     rbduckdb_init_appender();

ext/duckdb/pending_result.c

@@ -50,7 +50,7 @@ static VALUE pending_result_initialize(int argc, VALUE *argv, VALUE self) {
     }
 
     rubyDuckDBPendingResult *ctx = rbduckdb_get_struct_pending_result(self);
-    rubyDuckDBPreparedStatement *stmt = get_struct_prepared_statement(oDuckDBPreparedStatement);
+    rubyDuckDBPreparedStatement *stmt = rbduckdb_get_struct_prepared_statement(oDuckDBPreparedStatement);
 
     state = duckdb_pending_prepared(stmt->prepared_statement, &(ctx->pending_result));