GoogleAIClient and GoogleVertexClient were hardcoding Letta's managed
credentials for all requests, ignoring user-provided BYOK API keys.
This meant Letta was paying Google API costs for BYOK users.
Add _get_client_async and update _get_client to check BYOK overrides
(via get_byok_overrides / get_byok_overrides_async) before falling back
to managed credentials, matching the pattern used by OpenAIClient and
AnthropicClient.
🤖 Generated with [Letta Code](https://letta.com)
Co-authored-by: Letta <noreply@letta.com>
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