fix: store JSON custom parameters as strings instead of objects (#11501) (#11503)

Previously, JSON-type custom parameters were incorrectly parsed and stored
as objects in the UI layer, causing API requests to fail when getCustomParameters()
attempted to JSON.parse() an already-parsed object.

Changes:
- AssistantModelSettings.tsx: Remove JSON.parse() in onChange handler, store as string
- reasoning.ts: Add comments explaining JSON parsing flow
- BaseApiClient.ts: Add comments for legacy API clients

src/renderer/src/aiCore/legacy/clients/BaseApiClient.ts

@@ -405,6 +405,9 @@ export abstract class BaseApiClient<
         if (!param.name?.trim()) {
           return acc
         }
+        // Parse JSON type parameters (Legacy API clients)
+        // Related: src/renderer/src/pages/settings/AssistantSettings/AssistantModelSettings.tsx:133-148
+        // The UI stores JSON type params as strings, this function parses them before sending to API
         if (param.type === 'json') {
           const value = param.value as string
           if (value === 'undefined') {

src/renderer/src/aiCore/utils/reasoning.ts

@@ -684,6 +684,10 @@ export function getCustomParameters(assistant: Assistant): Record<string, any> {
       if (!param.name?.trim()) {
         return acc
       }
+      // Parse JSON type parameters
+      // Related: src/renderer/src/pages/settings/AssistantSettings/AssistantModelSettings.tsx:133-148
+      // The UI stores JSON type params as strings (e.g., '{"key":"value"}')
+      // This function parses them into objects before sending to the API
       if (param.type === 'json') {
         const value = param.value as string
         if (value === 'undefined') {

src/renderer/src/pages/settings/AssistantSettings/AssistantModelSettings.tsx

@@ -135,12 +135,18 @@ const AssistantModelSettings: FC<Props> = ({ assistant, updateAssistant, updateA
           <Input
             value={typeof param.value === 'string' ? param.value : JSON.stringify(param.value, null, 2)}
             onChange={(e) => {
-              try {
-                const jsonValue = JSON.parse(e.target.value)
-                onUpdateCustomParameter(index, 'value', jsonValue)
-              } catch {
-                onUpdateCustomParameter(index, 'value', e.target.value)
-              }
+              // For JSON type parameters, always store the value as a STRING
+              //
+              // Data Flow:
+              // 1. UI stores: { name: "config", value: '{"key":"value"}', type: "json" } ← STRING format
+              // 2. API parses: getCustomParameters() in src/renderer/src/aiCore/utils/reasoning.ts:687-696
+              //                calls JSON.parse() to convert string to object
+              // 3. Request sends: The parsed object is sent to the AI provider
+              //
+              // Previously this code was parsing JSON here and storing
+              // the object directly, which caused getCustomParameters() to fail when trying
+              // to JSON.parse() an already-parsed object.
+              onUpdateCustomParameter(index, 'value', e.target.value)
             }}
           />
         )