fix(Postgres Node): Stop marking autogenerated columns as required (#8230)

## Summary

Postgres columns can be
- [generated as
identity](https://www.postgresqltutorial.com/postgresql-tutorial/postgresql-identity-column/)
- [generated by a custom
expression](https://www.postgresql.org/docs/current/ddl-generated-columns.html)

In these 2 cases, the column is not required when inserting a new row.
This PR makes sure these types of column are not marked required in n8n.

### How to test

1. Create a Postgres table with all types of generated columns:
for version >= 10
  ```sql
  CREATE TABLE "public"."test_table" (
      "id" int8 NOT NULL DEFAULT nextval('test_table_id_seq'::regclass),
      "identity_id" bigint GENERATED ALWAYS AS IDENTITY,
      "id_plus" numeric GENERATED ALWAYS AS (id + 5) STORED,
      "title" varchar NOT NULL,
      "created_at" timestamp DEFAULT now(),
      PRIMARY KEY ("id")
  )
  ```
  Before 10 you have to use serial or bigserial types:
  ```sql
  CREATE TABLE distributors (
     did    serial not null primary key,
     name   varchar(40) NOT NULL CHECK (name <> '')
);
  ```
2. Add a postgres node to canvas and try to insert data without the
generated columns
3. Should successfully insert

More info in Linear/Github issue ⬇️ 

## Related tickets and issues
- fixes #7084
-
https://linear.app/n8n/issue/NODE-816/rmc-not-all-id-fields-should-be-required
-
https://linear.app/n8n/issue/NODE-681/postgres-cant-map-automatically-if-database-requires-a-field

## Review / Merge checklist
- [ ] PR title and summary are descriptive. **Remember, the title
automatically goes into the changelog. Use `(no-changelog)` otherwise.**
([conventions](https://github.com/n8n-io/n8n/blob/master/.github/pull_request_title_conventions.md))
- [ ] [Docs updated](https://github.com/n8n-io/n8n-docs) or follow-up
ticket created.
- [ ] Tests included.
> A bug is not considered fixed, unless a test is added to prevent it
from happening again.
   > A feature is not complete without tests.

---------

Co-authored-by: Michael Kret <michael.k@radency.com>

packages/nodes-base/nodes/Postgres/v2/helpers/utils.ts

@@ -376,13 +376,45 @@ export function prepareItem(values: IDataObject[]) {
 	return item;
 }
 
+export async function columnFeatureSupport(
+	db: PgpDatabase,
+): Promise<{ identity_generation: boolean; is_generated: boolean }> {
+	const result = await db.any(
+		`SELECT EXISTS (
+			SELECT 1 FROM information_schema.columns WHERE table_name = 'columns' AND table_schema = 'information_schema' AND column_name = 'is_generated'
+		) as is_generated,
+		EXISTS (
+			SELECT 1 FROM information_schema.columns WHERE table_name = 'columns' AND table_schema = 'information_schema' AND column_name = 'identity_generation'
+		) as identity_generation;`,
+	);
+
+	return result[0];
+}
+
 export async function getTableSchema(
 	db: PgpDatabase,
 	schema: string,
 	table: string,
+	options?: { getColumnsForResourceMapper?: boolean },
 ): Promise<ColumnInfo[]> {
+	const select = ['column_name', 'data_type', 'is_nullable', 'udt_name', 'column_default'];
+
+	if (options?.getColumnsForResourceMapper) {
+		// Check if columns exist before querying (identity_generation was added in v10, is_generated in v12)
+		const supported = await columnFeatureSupport(db);
+
+		if (supported.identity_generation) {
+			select.push('identity_generation');
+		}
+
+		if (supported.is_generated) {
+			select.push('is_generated');
+		}
+	}
+
+	const selectString = select.join(', ');
 	const columns = await db.any(
-		'SELECT column_name, data_type, is_nullable, udt_name, column_default FROM information_schema.columns WHERE table_schema = $1 AND table_name = $2',
+		`SELECT ${selectString} FROM information_schema.columns WHERE table_schema = $1 AND table_name = $2`,
 		[schema, table],
 	);