Don't pass type: :id to scope references in generated migrations

[eagle-head]

Summary

Fixes #6422.

The scope reference in the migration template (migration.exs.eex, line 10) was passing type: :id explicitly to references(). In Ecto's MySQL adapter, :id maps to integer (signed 32-bit), while the default PK is bigint unsigned auto_increment. This type mismatch causes:

(MyXQL.Error) (1005) (ER_CANT_CREATE_TABLE) Can't create table (errno: 150 "Foreign key constraint is incorrectly formed")

Ecto type mapping on MySQL

references() option	Ecto type	MySQL SQL	Matches default PK?
(no type)	:bigserial (default)	BIGINT UNSIGNED	Yes
type: :bigserial	:bigserial	BIGINT UNSIGNED	Yes
type: :id	:id	integer (32-bit)	No — errno 150

Before (broken on MySQL)

add :user_id, references(:users, type: :id, on_delete: :delete_all)

After (works on all adapters)

add :user_id, references(:users, on_delete: :delete_all)

By omitting type: when it's :id, Ecto defaults to :bigserial which correctly maps to BIGINT UNSIGNED on MySQL. Non-:id types like :binary_id are still passed explicitly.

Changes

• priv/templates/phx.gen.schema/migration.exs.eex — conditionally omit type: for scope references when schema_migration_type == :id
• Unit tests for both :id (omitted) and :binary_id (kept) scope types
• MySQL integration test with scoped resources verifying no errno 150