feat: add Gmail list messages sample

[emrila]

Description

Adds Gmail sample for listing messages and corresponding unit test. Based on the list_messages sample from Python.

Is it been tested?

• Development testing done

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have performed a peer-reviewed with team member(s)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[gemini-code-assist]

Summary of Changes

Hello, 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 introduces a new Java sample for interacting with the Gmail API to retrieve a list of messages from a user's inbox. It provides a clear, runnable example for developers, complete with necessary API client setup and robust unit tests to ensure its reliability and correct behavior under various conditions.

Highlights

• New Gmail Sample: A new Java sample has been added to demonstrate how to list messages from a user's Gmail inbox using the Gmail API.
• Unit Testing: A comprehensive unit test class has been included to verify the functionality of the new Gmail list messages sample, covering successful retrieval, error handling, and empty results.

Changelog

• gmail/snippets/src/main/java/ListMessages.java
  • Added a new Java class ListMessages to encapsulate the logic for listing Gmail messages.
  • Implemented the listMessages method to fetch up to 10 messages from the 'INBOX' label.
  • Included a getGmailService helper method for authenticating and building the Gmail API client.
• gmail/snippets/src/test/java/TestListMessages.java
  • Added a new JUnit test class TestListMessages to validate the ListMessages sample.
  • Implemented testListMessagesReturnsMessages to verify successful message retrieval with mock data.
  • Added testListMessagesHandlesApiError to ensure proper handling of API errors (e.g., 404 Not Found).
  • Included testListMessagesHandlesEmptyResults to confirm correct behavior when no messages are found.
  • Utilized MockHttpTransport for isolated and efficient testing of API interactions.

Activity

• The author has completed development testing for the changes.
• A self-review of the code has been performed by the author.
• Code has been commented, particularly in complex sections, to improve readability.
• The changes introduced do not generate any new warnings.

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[gemini-code-assist]

Code Review

This pull request introduces a new Gmail snippet for listing messages. The implementation has a critical performance issue due to making API calls within a loop, which should be addressed by using batch requests. Additionally, there are a couple of style guide violations related to wildcard imports in both the main code and the test file. I've provided specific suggestions to resolve these issues.