Add changelog for 2025-04-16

Author: quantstruct-bot

fern/changelog/2025-04-16.mdx

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+1. **Assistant Overrides in Testing (`TargetPlan.assistantOverrides`)**: You can now apply `assistantOverrides` when testing an assistant with a [Target Plan](https://api.vapi.ai/api#:~:text=TargetPlan), allowing modifications to the assistant's configuration specifically for tests without changing the original assistant. This helps in testing different configurations or behaviors of an assistant without affecting the live version.
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+2. **Specify Voice Model with Deepgram**: You can now specify the `model` to be used by Deepgram voices by setting the `model` property to `"aura"` or `"aura-2"` (default: `"aura-2"`).
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+3. **Expanded Deepgram Voice Options (`voiceId` in `DeepgramVoice` and `FallbackDeepgramVoice`)**: The list of available deepgram voice options has been greatly expanded, providing a wider selection of voices for assistants. This allows you to customize the assistant's voice to better match your desired persona with `Assistant.voice["DeepgramVoice"].voiceId`.
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+<Frame caption="Expanded Voice Options">
+    <img src="../static/images/changelog/deepgram-voices.png" alt="Expanded Deepgram Voice Options" />
+</Frame>
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+4. **Control Text Replacement Behavior (`replaceAllEnabled` in `ExactReplacement`)**: A new property `replaceAllEnabled` allows you to decide whether to replace all instances of a specified text (`key`) or just the first occurrence in [`ExactReplacement`](https://api.vapi.ai/api#:~:text=ExactReplacement) configurations. Setting `replaceAllEnabled` to `true` ensures that all instances are replaced.