O3-5428: Validate program enrollment/completion dates against patient DOB

[BlessedAmrita]

Description of what I changed

Added DOB lower-bound validation for dateEnrolled and dateCompleted in PatientProgramValidator.

When either date falls before the patient's date of birth, the validator now rejects it with a field error. This prevents clinically impossible data entry: a patient cannot be enrolled in a program before they were born. Dates equal to the birthdate are allowed.

Changes:

• PatientProgramValidator.java: Added birthdate lower-bound checks for both date fields after existing future-date checks, with null guards for patient and birthdate.
• PatientProgramValidatorTest.java: Added 10 test cases (5 per date field) covering before/equal/after birthdate, null date, and null birthdate scenarios
• messages.properties: Added error.patientProgram.enrolledDateCannotBeBeforeBirthdate and error.patientProgram.completionDateCannotBeBeforeBirthdate keys

Test scenarios (for each date field):

• Date before patient birthdate → validation error
• Date equal to patient birthdate → no error
• Date after patient birthdate → no error
• Date is null → no error
• Patient birthdate is null → no error

Part of the Date Picker Improvements epic (O3-5424).

Issue I worked on

see https://issues.openmrs.org/browse/O3-5428

Checklist: I completed these to help reviewers :)

• My IDE is configured to follow the code style of this project.
• I have added tests to cover my changes. (If you refactored
  existing code that was well tested you do not have to add tests)
• I ran mvn clean package right before creating this pull request and
  added all formatting changes to my commit.
• All new and existing tests passed.
• My pull request is based on the latest changes of the master branch.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[BlessedAmrita]

@denniskigen @dkayiwa please review this

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.25%. Comparing base (dcf1695) to head (178c6ed).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...org/openmrs/validator/PatientProgramValidator.java	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5886      +/-   ##
============================================
- Coverage     59.26%   59.25%   -0.01%     
- Complexity     9251     9260       +9     
============================================
  Files           686      686              
  Lines         37219    37225       +6     
  Branches       5445     5448       +3     
============================================
+ Hits          22056    22057       +1     
- Misses        13201    13207       +6     
+ Partials       1962     1961       -1     

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