🏷️ fix: Clear ModelSpec Display Fields When Navigating via Agent Share Link (#12274)

- Extract `specDisplayFieldReset` constant and `mergeQuerySettingsWithSpec`
  utility to `client/src/utils/endpoints.ts` as a single source of truth
  for spec display fields that must be cleared on non-spec transitions.
- Clear `spec`, `iconURL`, `modelLabel`, and `greeting` from the merged
  preset in `ChatRoute.getNewConvoPreset()` when URL query parameters
  override the conversation without explicitly setting a spec.
- Also clear `greeting` in the parallel cleanup in `useQueryParams.newQueryConvo`
  using the shared `specDisplayFieldReset` constant.
- Guard the field reset on `specPreset != null` so null values aren't
  injected when no spec is configured.
- Add comprehensive test coverage for the merge-and-clear logic.

client/src/hooks/Input/useQueryParams.ts

@@ -12,6 +12,7 @@ import type {
 import {
   clearModelForNonEphemeralAgent,
   removeUnavailableTools,
+  specDisplayFieldReset,
   processValidSettings,
   getModelSpecIconURL,
   getConvoSwitchLogic,
@@ -128,13 +129,10 @@ export default function useQueryParams({
         endpointsConfig,
       });
 
-      let resetParams = {};
+      const resetFields = newPreset.spec == null ? specDisplayFieldReset : {};
       if (newPreset.spec == null) {
-        template.spec = null;
-        template.iconURL = null;
-        template.modelLabel = null;
-        resetParams = { spec: null, iconURL: null, modelLabel: null };
-        newPreset = { ...newPreset, ...resetParams };
+        Object.assign(template, specDisplayFieldReset);
+        newPreset = { ...newPreset, ...specDisplayFieldReset };
       }
 
       // Sync agent_id from newPreset to template, then clear model if non-ephemeral agent
@@ -152,7 +150,7 @@ export default function useQueryParams({
           conversation: {
             ...(conversation ?? {}),
             endpointType: template.endpointType,
-            ...resetParams,
+            ...resetFields,
           },
           preset: template,
           cleanOutput: newPreset.spec != null && newPreset.spec !== '',