Switch to usageHint from hint

[gspencergoog]

Description

Switches the name of the hint property to be usageHint, which describes it in less generic terms.

[gemini-code-assist]

Code Review

This pull request correctly refactors the hint property to usageHint across multiple files, improving clarity. The changes are consistent and cover code, tests, and schema definitions. I've added a few suggestions to further improve code maintainability and documentation by addressing magic numbers and enhancing doc comments.

[gemini-code-assist]

The description for usageHint in the schema provides useful details about what each hint value means. It would be beneficial to include this information in the documentation comment here as well to improve discoverability for developers reading the code.

Suggestion:

/// - `usageHint`: A usage hint for the image size and style. One of:
///   - `icon`: Small square icon.
///   - `avatar`: Circular avatar image.
///   - `smallFeature`: Small feature image.
///   - `mediumFeature`: Medium feature image.
///   - `largeFeature`: Large feature image.
///   - `header`: Full-width, full bleed, header image.

[gemini-code-assist]

The documentation for usageHint could be more descriptive. Providing details for each hint value, similar to what's in packages/flutter_genui_a2ui/server_to_client.json, would be helpful for developers.

Suggestion:

/// - `usageHint`: A usage hint for the text size and style. One of:
///   - `h1`: Largest heading.
///   - `h2`: Second largest heading.
///   - `h3`: Third largest heading.
///   - `h4`: Fourth largest heading.
///   - `h5`: Fifth largest heading.
///   - `caption`: Small text for captions.
///   - `body`: Standard body text.