Refine connector provider registry per review

Three follow-ups from review of the registry consolidation:

- Build Vercel's authorization URL with net/url instead of a
  hand-rolled "{integration_slug}" placeholder resolved by
  strings.ReplaceAll. The slug is escaped via url.PathEscape in a
  per-provider Registration.BuildAuthURL closure, and the unused
  AuthURLParams plumbing on Registration and OAuth2Connector is
  removed (OAuth2Connector now carries a typed IntegrationSlug).

- Drop the SettingsInput union type and the per-provider
  MarshalSettings closures. The create resolvers now build the typed
  coredata.*ConnectorSettings directly from the gqlgen input, the
  same way the OAuth callback path already does, so there is no
  shared catch-all DTO and no stringly-typed boundary.

- Restore ConnectorProviders() to a plain ordered slice literal; the
  intermediate map + slices.Sort added nondeterminism and a sort for
  no benefit.

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

pkg/probod/probod.go

@@ -288,7 +288,9 @@ func (impl *Implm) Run(
 
 	for _, connectorCfg := range impl.cfg.Connectors {
 		if oauth2c, ok := connectorCfg.Config.(*connector.OAuth2Connector); ok {
-			providerRegistry.ApplyOAuth2Defaults(connectorCfg.Provider, redirectURI, oauth2c)
+			if err := providerRegistry.ApplyOAuth2Defaults(connectorCfg.Provider, redirectURI, oauth2c); err != nil {
+				return fmt.Errorf("cannot apply oauth2 defaults: %w", err)
+			}
 		}
 
 		if err := defaultConnectorRegistry.Register(connectorCfg.Provider, connectorCfg.Config); err != nil {