feat: model clean-up

[SergeyMenshykh]

This PR aligns the names, types, and nullability of properties in model classes with those from the specification. It also replaces all instances of the RequiredAttribute with the JsonRequiredAttribute.

[gemini-code-assist]

Summary of Changes

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This pull request focuses on a significant clean-up of the model classes, primarily by aligning their property definitions (names, types, and nullability) with the project's specification. A key part of this effort involves migrating from the RequiredAttribute to the JsonRequiredAttribute for JSON serialization validation, ensuring that required properties are correctly enforced during JSON processing.

Highlights

• Model Property Alignment: Properties across various model classes have been updated to match the specification, including changes from nullable types (e.g., string?, List<T>?, Dictionary<K,V>?) to non-nullable types with appropriate default initializations (e.g., string.Empty, new List<T>(), new Dictionary<K,V>()).
• JSON Serialization Validation: The System.ComponentModel.DataAnnotations.RequiredAttribute has been systematically replaced with System.Text.Json.Serialization.JsonRequiredAttribute in model classes, ensuring that JSON serialization and deserialization correctly enforce required fields.
• TaskPushNotificationConfig Renaming: The Id property within the TaskPushNotificationConfig model has been renamed to TaskId to improve clarity and consistency, with all references updated accordingly in the codebase, including the A2AHttpProcessor, InMemoryTaskStore, and various unit tests.
• Sample HTTP Request Updates: Minor adjustments were made to the sample HTTP request files to reflect changes in API method names (e.g., task/get to tasks/get) and updated comment formatting.

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[gemini-code-assist]

Code Review

This pull request cleans up the data models to align with the specification by adjusting property names, types, nullability, and replacing RequiredAttribute with JsonRequiredAttribute. The changes are reflected in the updated tests. The model for FilePart seems to have become overly restrictive, no longer supporting file references by URI. Otherwise, the changes look good.