Fix: Retrieve the most recent messages in Mail Reader Sampler

[tolgagul]

Description

This update addresses an issue in the Mail Reader Sampler where the default behavior retrieves the oldest emails instead of the most recent ones. The fix modifies the getMessages method to correctly fetch the last n messages from the mailbox.

Adjusted the indices in the folder.getMessages() call to dynamically calculate the range for the most recent messages.
Added error handling for cases where n exceeds the total number of messages.

Motivation and Context

This change is required to resolve a common issue with the Mail Reader Sampler, where users typically need the most recent emails rather than the oldest ones. By implementing this fix:

• Users can retrieve the latest n emails directly.
• Aligns the sampler's behavior with user expectations and IMAP usage patterns.
  This PR addresses Issue Retrieving the Most Recent Emails in Mail Reader Sampler #6384

How Has This Been Tested?

1. Test Scenarios:

• Mailbox with fewer than n emails: Verified that all available emails are fetched.
• Mailbox with more than n emails: Confirmed that only the last n messages are retrieved.
• Boundary conditions: Tested with n=1, n=total_message_count, and n=total_message_count + 1.

2. Environment:

• Tested on a Gmail IMAP mailbox and a local test IMAP server.
• Verified compatibility with both SSL and non-SSL connections.

3. Results:

• All test cases passed successfully.
• No observed regressions in functionality.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.