inner loop, update changelog script, add unit test (#47373)

* update changelog, Add Unit test

* rename file

* rename the module file

* add a pipeline

* remove module, use helper functions instead

* delete the ci file

* Add realistic test cases for changelog automation

- Test changelog updates when source code changes
- Test idempotent behavior (multiple runs produce identical output)
- Validate unreleased section is preserved after updates
- Remove mocks to test actual file update behavior

Resolves: #47373 (comment)

Author: haolingdong-msft