Add Sequence Emails CRUD Methods

[n7studios]

Summary

Adds support for the Sequences CRUD methods, which were added to the API on May 8th 2026:
https://developers.kit.com/changelog#may-8-2026

Testing

• testGetSequenceEmails — Test that get_sequence_emails() returns the expected data.
• testGetSequenceEmailsWithTotalCount — Test that get_sequence_emails() returns the expected data when the total count is included.
• testGetSequenceEmailsPagination — Test that get_sequence_emails() returns the expected data when pagination parameters and per_page limits are specified.
• testGetSequenceEmailsWithInvalidSequenceID — Test that get_sequence_emails() throws a ClientException when an invalid sequence ID is specified.
• testCreateGetUpdateAndDeleteSequenceEmail — Test that create_sequence_email(), get_sequence_email(), update_sequence_email() and delete_sequence_email() works.
• testGetSequenceEmailWithInvalidSequenceID — Test that get_sequence_email() throws a ClientException when an invalid sequence ID is specified.
• testGetSequenceEmailWithInvalidEmailID — Test that get_sequence_email() throws a ClientException when an invalid email ID is specified.
• testUpdateSequenceEmailWithInvalidSequenceID — Test that update_sequence_email() throws a ClientException when an invalid sequence ID is specified.
• testUpdateSequenceEmailWithInvalidEmailID — Test that update_sequence_email() throws a ClientException when an invalid email ID is specified.
• testDeleteSequenceEmailWithInvalidSequenceID — Test that delete_sequence_email() throws a ClientException when an invalid sequence ID is specified.
• testDeleteSequenceEmailWithInvalidEmailID — Test that delete_sequence_email() throws a ClientException when an invalid email ID is specified.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)