chore: Centralise PageToken parsing

[kabir]

fixes: #597

[gemini-code-assist]

Summary of Changes

Hello @kabir, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

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.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable refactoring by centralizing the logic for parsing and generating page tokens into a new PageToken utility class. This change effectively removes duplicated code from JpaDatabasePushNotificationConfigStore, JpaDatabaseTaskStore, and InMemoryTaskStore, which significantly improves code clarity and maintainability. The new implementation also enforces stricter validation on page tokens, correctly throwing an error for invalid formats, which is a good improvement for robustness. The accompanying test updates accurately reflect this new behavior. I have one suggestion to further improve the validation in the new PageToken class.