O3-5428: Validate program enrollment dates against patient DOB

[BlessedAmrita]

Description of what I changed

Added DOB lower-bound validation in ProgramEnrollmentResource1_8 to reject dateEnrolled and dateCompleted values that fall before the patient's date of birth.

• Added a new create() override to validate dateEnrolled and dateCompleted against the patient's birthdate before delegating to super.create()
• Added a new update() override to validate both dateEnrolled and dateCompleted against the patient's birthdate before delegating to super.update()
• Throws IllegalRequestException (HTTP 400) with a clear error message
• Null-safe: skips validation if the patient's birthdate is null or if the date fields are absent from the request
• Validation is in ProgramEnrollmentResource1_8 (the base class) so 1_10 and 2_2 subclasses inherit it automatically
• Added 5 tests covering rejection (create/update with dates before DOB) and acceptance (valid dates)

Issue I worked on

see https://openmrs.atlassian.net/browse/O3-5428

Checklist: I completed these to help reviewers :)

• My IDE is configured to follow the code style of this project.

  No? Unsure? -> configure your IDE, format the code and add the changes with git add . && git commit --amend

• I have added tests to cover my changes. (If you refactored
  existing code that was well tested you do not have to add tests)

  No? -> write tests and add them to this commit git add . && git commit --amend

• I ran mvn clean package right before creating this pull request and
  added all formatting changes to my commit.

  No? -> execute above command

• All new and existing tests passed.

  No? -> figure out why and add the fix to your commit. It is your responsibility to make sure your code works.

• My pull request is based on the latest changes of the master branch.

  No? Unsure? -> execute command git pull --rebase upstream master

[BlessedAmrita]

@denniskigen, please review this