fix: Add Support for Proper Image Size Constraints

[sfreeman422]

This pull request refactors the handling of image size configuration across several API and type definition files. The main improvement is the introduction of a dedicated ImageSizeConfig type, replacing the previous use of generic strings for specifying image sizes. This change enhances type safety and ensures that only supported image sizes can be used in the relevant interfaces.

Type safety improvements for image size configuration:

• Introduced a new ImageSizeConfig type ('1K' | '2K' | '4K') to represent valid image sizes, replacing the previous untyped string usage in image-related configuration interfaces. ([[1]](https://github.com/googleapis/js-genai/pull/1413/files#diff-5e6f47503bac0a3672b747dd68d8cad573b5e0d78c77b7a659488d98d39a93b5R1499-R1500), [[2]](https://github.com/googleapis/js-genai/pull/1413/files#diff-c54113cf61ec99691748a3890bfbeb00e10efb3f0a76f03a0fd9ec49072e410aR2348-R2349))
• Updated the imageSize property in the ImageConfig interface to use ImageSizeConfig instead of string in both the TypeScript types and the generated API report files. ([[1]](https://github.com/googleapis/js-genai/pull/1413/files#diff-5e6f47503bac0a3672b747dd68d8cad573b5e0d78c77b7a659488d98d39a93b5L2008-R2010), [[2]](https://github.com/googleapis/js-genai/pull/1413/files#diff-20ae5185c323ce5cf41072fbd950bfa72b92eb8d3ccd8776c085b477bb361b6eL2008-R2008), [[3]](https://github.com/googleapis/js-genai/pull/1413/files#diff-c54113cf61ec99691748a3890bfbeb00e10efb3f0a76f03a0fd9ec49072e410aL2356-R2358))
• Updated the imageSize property in the GenerateImagesConfig interface to use ImageSizeConfig (or a version omitting '4K' where not supported), replacing the previous string type in both the TypeScript types and the generated API report files. ([[1]](https://github.com/googleapis/js-genai/pull/1413/files#diff-5e6f47503bac0a3672b747dd68d8cad573b5e0d78c77b7a659488d98d39a93b5L1507-R1509), [[2]](https://github.com/googleapis/js-genai/pull/1413/files#diff-20ae5185c323ce5cf41072fbd950bfa72b92eb8d3ccd8776c085b477bb361b6eL1507-R1507), [[3]](https://github.com/googleapis/js-genai/pull/1413/files#diff-c54113cf61ec99691748a3890bfbeb00e10efb3f0a76f03a0fd9ec49072e410aL3487-R3489))

These changes make the codebase more robust by preventing invalid image size values at compile time and clarifying the supported options for image generation.

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