Summary of Changes

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This pull request refactors the application's pagination token handling by introducing a dedicated PageToken utility class. This change centralizes the logic for parsing and generating pagination tokens, ensuring consistent validation and error reporting across different data store implementations. The aim is to improve code maintainability and robustness by eliminating duplicated parsing logic and standardizing how malformed tokens are handled.

Highlights

• New PageToken Utility Class: Introduced a new PageToken record class in io.a2a.util to encapsulate the parsing and generation logic for pagination tokens, standardizing the format 'timestamp_millis:id'.
• Centralized PageToken Parsing and Validation: Refactored existing code in JpaDatabasePushNotificationConfigStore, JpaDatabaseTaskStore, and InMemoryTaskStore to utilize the new PageToken.fromString() method for parsing, centralizing error handling for invalid token formats to throw InvalidParamsError.
• Consistent PageToken Generation: Updated the generation of nextPageToken across all affected stores to use the PageToken object's toString() method, ensuring uniformity.
• Updated Test Cases: Modified integration tests in JpaDatabasePushNotificationConfigStoreIntegrationTest to reflect the new behavior where invalid page tokens now consistently result in an InvalidParamsError.