Add changelog for 2025-08-27 (#647)

Author: quantstruct-bot

fern/changelog/2025-08-27.mdx

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+1. **Enhanced Tool Retry Logic with Backoff Plans**: You can now use [`Assistant.hooks.do[type=tool].tool.backoffPlan`](https://api.vapi.ai/api#:~:text=CartesiaTranscriber-,BackoffPlan,-%7B) and [`Assistant.hooks.do[type=tool].tool.server.backoffPlan`](https://api.vapi.ai/api#:~:text=CartesiaTranscriber-,BackoffPlan,-%7B) to configure retry behavior for tool calls. Options include:
+
+- **`fixed` backoff** (default): Consistent delay between retries.
+- **`exponential` backoff**: Increasing delays for subsequent retries
+- **Configurable retry limits**: Set `maxRetries` (0-10, default: 0)
+- **Flexible timing**: Adjust `baseDelaySeconds` (0-10 seconds)
+- **Smart status code handling**: Exclude specific HTTP status codes from retry attempts.