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/connector/oauth2.go

@@ -57,12 +57,13 @@ type (
 		// to the authorize URL; CompleteWithState replays the verifier
 		// on the token exchange.
 		RequiresPKCE bool
-		// AuthURLParams are operator-supplied placeholders substituted
-		// into the static provider AuthURL by
-		// (*provider.Registry).ApplyOAuth2Defaults (for example
-		// Vercel's "{integration_slug}"). Empty for the vast majority
-		// of providers.
-		AuthURLParams map[string]string
+		// IntegrationSlug is an operator-supplied identifier used by
+		// providers whose authorization URL embeds it as a path segment
+		// (Vercel-style integrations). It is consumed by the provider's
+		// Registration.BuildAuthURL in
+		// (*provider.Registry).ApplyOAuth2Defaults. Empty for the vast
+		// majority of providers.
+		IntegrationSlug string
 
 		// HTTPClient is used for the OAuth2 token-exchange request
 		// issued from CompleteWithState. It must be set by callers;