feat: course metadata versioning and get_platform_fee access control …

[EDOHWARES]

Summary of Changes

This PR resolves issues #67 (Course Metadata Versioning) and #70 (get_platform_fee() Access Control).

1. Course Metadata Versioning (#67)

• Course Struct: Added version: u32 field (starts at 1 upon registration).
• Enrollment Struct: Added course_version: u32 field to immutably record the active course version at the time of enrollment (populated in both enroll() and re_enroll()).
• update_course() Method: Added update_course(env, caller, course_id, new_price, new_max_capacity) in lib.rs allowing authorized instructors or admins to update course details while incrementing course.version.

2. Access Control for get_platform_fee() (#70)

• Admin Authentication: Updated get_platform_fee(env: Env, admin: Address) -> u32 to require admin signature (admin.require_auth() and require_admin).
• Security Model Documentation: Updated the contract header security model in lib.rs documenting the access control rationale to prevent business intelligence exposure.

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Task Checklist

• Add version: u32 field to Course struct in contracts/hamplard/src/lib.rs
• Add course_version: u32 field to Enrollment struct in contracts/hamplard/src/lib.rs
• Populate enrollment version in enroll() and re_enroll()
• Implement update_course() function incrementing version on course updates
• Restrict get_platform_fee() to admin-only access
• Document get_platform_fee() access control in contract security model
• Update existing tests and add unit tests verifying version tracking and fee access control

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Test Verification

• Executed full Soroban Rust test suite via cargo test:
  test result: ok. 123 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
• Added test coverage for:
  • test_course_metadata_versioning_across_updates
  • test_get_platform_fee_unauthorized_fails
  • test_update_course_unauthorized_fails

Closes #67
Closes #70
Closes #69
Closes #105

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[EDOHWARES]

@SYLVIANNORUKA , pls review PR