Address code review feedback

- Return 400 instead of panicking on invalid organization_id
- Use generic error message for internal failures
- Drop duplicate validation from initiate handler (kept in tx)
- Make preserveConnectionFields mutate in place
- Remove as type assertions in GoogleWorkspaceConnector
- Use sort.Slice instead of sort.SliceStable
- Consistent error prefixes in Slack sender

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/coredata/connector.go

@@ -147,7 +147,7 @@ func (c *Connector) LoadOneByOrganizationIDAndProvider(
 	}
 
 	// Widest-scope-wins, tiebreak by most recent updated_at.
-	sort.SliceStable(connectors, func(i, j int) bool {
+	sort.Slice(connectors, func(i, j int) bool {
 		ci, cj := connectorScopeCount(connectors[i]), connectorScopeCount(connectors[j])
 		if ci != cj {
 			return ci > cj