`mediaResolution` parameter missing from GenerationConfig

**Description**
I'm trying to use the `mediaResolution` parameter as described in the Vertex AI REST API documentation, but I’ve noticed that this library does not currently support this field. Specifically, the `GenerationConfig` does not include the `mediaResolution` parameter, which makes it impossible to fully utilize the API features through the Node.js client.

**Suggested fix**
Please update the `GenerationConfig` type definition in the library to match the actual REST API capabilities.

**Additional context**
Here’s the relevant documentation for the `mediaResolution` parameter:
https://ai.google.dev/api/generate-content#MediaResolution

Locate the GenerationConfig type definition in the TypeScript Node.js client and compare it with the mediaResolution details in the linked Vertex AI REST API documentation. Confirm how the existing generation configuration fields are represented, then verify that the type supports this documented parameter and that relevant type checks or tests pass.

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