chore: add changelog for 2025-06-23 with latest OpenAPI breaking changes

Author: quantstruct-bot

fern/changelog/2025-06-23.mdx

@@ -0,0 +1,41 @@
+---
+title: Changelog for 2025-06-23
+---
+
+# Changelog — 2025-06-23
+
+## Breaking Changes
+
+- **Schema Removals**:
+    - `call.workflow.nodes[type=conversation].model` has been removed from both `WorkflowCustomModel` and `WorkflowGoogleModel`. You must now specify model configuration at the appropriate node level.
+    - The following properties were removed from schemas:
+        - `Chat`: `assistantOverrides`
+        - `McpTool`: `metadata`
+        - `Campaign`: `callsCounterQueued`, `callsCounterScheduled`, `callsCounterInProgress`, `callsCounterEndedVoicemail`
+        - `LMNTVoice` and `FallbackLMNTVoice`: `language`
+        - `CreateChatDTO` and `CreateMcpToolDTO`: `assistantOverrides`, `metadata`
+        - `UpdateMcpToolDTO`: `metadata`
+        - `OpenAIResponsesRequest`: `assistantOverrides`
+
+- **Voice Model Changes**:
+    - The `language` property has been removed from both `LMNTVoice` and `FallbackLMNTVoice`. Specify language at a higher level or use the new configuration pattern if needed.
+
+- **Campaign API Changes**:
+    - Campaign schemas no longer track call counters (`callsCounterQueued`, etc.) directly. Use new analytics endpoints or properties for campaign metrics.
+
+## Migration Guidance
+
+- **Workflows**: Ensure all model configuration is set at the node level, not at the workflow root or deprecated locations.
+- **Voice**: Update any references to the `language` property in LMNTVoice or FallbackLMNTVoice to use the new recommended configuration.
+- **Campaigns**: Migrate off direct call counter properties and use the analytics or reporting endpoints for campaign metrics.
+- **AssistantOverrides**: Remove any usage of the deprecated `assistantOverrides` property in chat or tool creation payloads.
+
+## Developer Notes
+
+- These changes are part of ongoing schema simplification and improved consistency.
+- If you use MCP tools, note that the `metadata` property is now removed from both the tool and creation/update DTOs.
+- For OpenAI tool integrations, ensure `assistantOverrides` is not used in the request payloads.
+
+---
+
+**For questions or migration support, please contact the Vapi developer support team or visit the latest [API Reference](../reference).**