fix(schema): resolve broken $ref URLs across schemas

[AbhiPra24]

Summary

Fixes broken relative and absolute $ref pointers across schema definitions in the repository to ensure all referenced schema fragments and files resolve cleanly without 404 or missing file errors.

Fixes #56

Changes

1. CallbackAction/v2.0 & RequestAction/v2.0:

   • Removed broken $ref pointers targeting non-existent BecknAction/v2.0 schemas in both attributes.yaml and schema.json.
   • Explicitly marked them as type: object deprecated stubs.

2. TransactionEndpoint/v2.0 & LineageEntry/v2.0:

   • Removed invalid allOf inheritance pointing to non-existent BecknEndpoint/v2.0 from TransactionEndpoint/v2.0 (attributes.yaml and schema.json).
   • Defined TransactionEndpoint/v2.0 directly as type: string with standard endpoint enumeration.
   • Updated LineageEntry/v2.0 action property $ref in attributes.yaml, schema.json, and README.md to point to TransactionEndpoint/v2.0.

3. GroceryItem/v2.0 & HomeAndKitchenItem/v2.0:

   • Removed invalid allOf inheritance pointing to deleted RetailCoreItem/v2.0 from attributes.yaml and schema.json.
   • Defined both item attribute schemas directly as type: object.

4. SalesOfferPackage/v2.0:

   • Updated distributions property $ref in attributes.yaml, schema.json, and README.md to point to existing CommunicationChannel/v2.0 instead of non-existent DistributionChannel/v2.0.

5. SafetyPolicy/v2.0:

   • Updated emergencyContact property $ref in attributes.yaml, schema.json, and README.md to point to existing Contact/v2.0 instead of non-existent ContactHandle/v2.0.

6. CodedValue/v2.1 & CourseResourceAttributes/v2.1:

   • Generated missing schema/CodedValue/v2.1/schema.json to satisfy the $ref targets in CourseResourceAttributes/v2.1/schema.json.

7. Documentation & Changelog:

   • Updated CHANGELOG.md with an unreleased patch record detailing the fixes.

Verification

• Ran full automated repository scan verifying all 878+ $ref pointers across all YAML and JSON schemas.
• 0 YAML parse errors, 0 JSON parse errors, 0 unresolved $ref errors.