fix: remove unique constraint on OAuth2 provider app names (#18669)

# Remove unique constraint on OAuth2 provider app names

This PR removes the unique constraint on the `name` field in the `oauth2_provider_apps` table to comply with RFC 7591, which only requires unique client IDs, not unique client names.

Changes include:
- Removing the unique constraint from the database schema
- Adding migration files for both up and down migrations
- Removing the name uniqueness check in the in-memory database implementation
- Updating the unique constraint constants

Change-Id: Iae7a1a06546fbc8de541a52e291f8a4510d57e8a
Signed-off-by: Thomas Kosiewski <tk@coder.com>

coderd/database/dbmem/dbmem.go

@@ -8983,12 +8983,6 @@ func (q *FakeQuerier) InsertOAuth2ProviderApp(_ context.Context, arg database.In
 	q.mutex.Lock()
 	defer q.mutex.Unlock()
 
-	for _, app := range q.oauth2ProviderApps {
-		if app.Name == arg.Name {
-			return database.OAuth2ProviderApp{}, errUniqueConstraint
-		}
-	}
-
 	//nolint:gosimple // Go wants database.OAuth2ProviderApp(arg), but we cannot be sure the structs will remain identical.
 	app := database.OAuth2ProviderApp{
 		ID:                      arg.ID,