refactor(coderd/x/chatd): remove dead model-routing dispatch shim (#26942)

Follow-up to #26862 ("remove direct chat routing"), which collapsed the
routing discriminated union into a single `aiGatewayModelRoute` but left
a one-path dispatch shim behind in `model_routing.go`.

Removes
`resolveModelRouteForConfig`/`resolveModelRouteForProviderType`/`newModel`
wrapper functions that did nothing but call their `*AIGateway*`
counterparts, and renames the `*AIGateway*` targets to take over those
names directly. Also collapses a redundant if/else in
`title_override.go` where both branches called the same function with
the same effective argument, and has `chatutil.NormalizedStringPointer`
delegate to the existing `coderd/util/strings.EmptyToNil` instead of
reimplementing empty-string-to-nil logic.

No behavior change.

<details>
<summary>Investigation notes / decision log</summary>

Two independent read-only investigations were run over `coderd/x/chatd`
looking for cleanup opportunities following #26862: one focused on
residue from that PR specifically, one a general over-engineering pass
on the whole package. Both independently converged on the
`model_routing.go` shim as the top finding (verified zero divergent call
sites).

Other candidates considered and explicitly deferred/rejected for this
PR:

- Renaming away the vestigial `AIGateway` prefix package-wide:
cosmetic-only, touches many call sites, skipped.
- Inlining the `chatcost` subpackage into `chatd`: unrelated to #26862,
skipped.
- Deleting the deprecated `AIGatewayRoutingEnabled` deployment flag:
confirmed dead/no-op, but intentionally kept as a back-compat shim per
#26862; removal should follow the same deprecation cadence as other
deprecated deployment options, as a separate, differently-timed change.
- Folding `chatutil` entirely into `chatprovider`/`chatopenai`:
`NormalizedStringPointer` overlapped with
`coderd/util/strings.EmptyToNil` (now reused), but `NormalizedEnumValue`
has no equivalent elsewhere in the repo and still has 2 real call sites,
so the package stays.

</details>

---
Generated by Coder Agents on behalf of @johnstcn.

coderd/x/chatd/chatutil/chatutil.go

@@ -1,6 +1,10 @@
 package chatutil
 
-import "strings"
+import (
+	"strings"
+
+	stringutil "github.com/coder/coder/v2/coderd/util/strings"
+)
 
 // NormalizedStringPointer trims a string pointer and returns nil for nil or
 // empty values.
@@ -8,11 +12,7 @@ func NormalizedStringPointer(value *string) *string {
 	if value == nil {
 		return nil
 	}
-	trimmed := strings.TrimSpace(*value)
-	if trimmed == "" {
-		return nil
-	}
-	return &trimmed
+	return stringutil.EmptyToNil(strings.TrimSpace(*value))
 }
 
 // NormalizedEnumValue returns the canonical allowed value matching value after

coderd/x/chatd/model_routing.go

@@ -52,31 +52,6 @@ func (p *Server) enabledAIProviderByID(ctx context.Context, providerID uuid.UUID
 	return provider, nil
 }
 
-func (p *Server) resolveModelRouteForConfig(
-	ctx context.Context,
-	ownerID uuid.UUID,
-	modelConfig database.ChatModelConfig,
-) (aiGatewayModelRoute, error) {
-	return p.resolveAIGatewayModelRouteForConfig(ctx, ownerID, modelConfig)
-}
-
-func (p *Server) resolveModelRouteForProviderType(
-	ctx context.Context,
-	ownerID uuid.UUID,
-	providerType string,
-) (aiGatewayModelRoute, error) {
-	return p.resolveAIGatewayModelRouteForProviderType(ctx, ownerID, providerType)
-}
-
-func (p *Server) newModel(
-	ctx context.Context,
-	req modelClientRequest,
-	route aiGatewayModelRoute,
-	opts modelBuildOptions,
-) (fantasy.LanguageModel, error) {
-	return p.newAIGatewayModel(ctx, req, route, opts)
-}
-
 func newLanguageModel(
 	providerHint string,
 	modelName string,

coderd/x/chatd/model_routing_aibridge.go

@@ -110,7 +110,7 @@ func isOpenRouterLikeAIGatewayProvider(provider database.AIProvider) bool {
 	return host == "openrouter.ai" || strings.HasSuffix(host, ".openrouter.ai")
 }
 
-func (p *Server) newAIGatewayModel(
+func (p *Server) newModel(
 	_ context.Context,
 	req modelClientRequest,
 	route aiGatewayModelRoute,
@@ -267,7 +267,7 @@ func (p *Server) resolveAIGatewayRoute(
 	return newAIGatewayModelRoute(provider, modelProviderHint, auth), nil
 }
 
-func (p *Server) resolveAIGatewayModelRouteForConfig(
+func (p *Server) resolveModelRouteForConfig(
 	ctx context.Context,
 	ownerID uuid.UUID,
 	modelConfig database.ChatModelConfig,
@@ -279,7 +279,7 @@ func (p *Server) resolveAIGatewayModelRouteForConfig(
 	return p.resolveAIGatewayRoute(ctx, ownerID, provider, string(provider.Type))
 }
 
-func (p *Server) resolveAIGatewayModelRouteForProviderType(
+func (p *Server) resolveModelRouteForProviderType(
 	ctx context.Context,
 	ownerID uuid.UUID,
 	providerType string,

coderd/x/chatd/title_override.go

@@ -54,18 +54,7 @@ func (p *Server) resolveTitleGenerationModelOverride(
 		chat.OwnerID,
 		p.resolveModelConfigAndNormalizedProvider,
 		func(ctx context.Context, ownerID uuid.UUID, aiProviderID uuid.UUID) (chatprovider.ProviderAPIKeys, error) {
-			if aiProviderID == uuid.Nil {
-				resolvedProviderKeys, err := p.resolveUserProviderAPIKeys(ctx, ownerID, uuid.Nil)
-				if err != nil {
-					return chatprovider.ProviderAPIKeys{}, err
-				}
-				return resolvedProviderKeys, nil
-			}
-			resolvedProviderKeys, err := p.resolveUserProviderAPIKeys(ctx, ownerID, aiProviderID)
-			if err != nil {
-				return chatprovider.ProviderAPIKeys{}, err
-			}
-			return resolvedProviderKeys, nil
+			return p.resolveUserProviderAPIKeys(ctx, ownerID, aiProviderID)
 		},
 		modelOverrideFailureModeHard,
 	)