Add priority to variant-mapping.csv

AI-assisted change in packages/cli-supplier-reports
Added variant_priority to the generated variant-mapping.csv output
Defaulted missing variant priority values to 50 to match the letter variant schema
Updated unit tests in packages/cli-supplier-reports/src/__tests__/supplier-report.test.ts
Added coverage for explicit priority output
Added coverage for default priority output when omitted
Updated CSV header expectations
Updated packages/cli-supplier-reports/README.md
Documented variant_priority
Aligned docs with the existing pack_specification_billing_id column
Validation completed:
Passed package unit tests
Passed markdown format check for the updated README
Passed pre-commit hooks for the changed files
Passed a CLI spot-check using the repository specifications.xlsx

Author: m-houston

packages/cli-supplier-reports/README.md

@@ -71,10 +71,12 @@ The variant mapping CSV includes the following columns:
 - `variant_id` - Letter variant ID
 - `variant_name` - Letter variant name
 - `variant_status` - Letter variant environment status (INT/PROD)
+- `variant_priority` - Letter variant dispatch priority (1 to 99, lower values are higher priority)
 - `pack_specification_id` - Pack specification ID
 - `pack_specification_name` - Pack specification name
 - `pack_specification_status` - Pack specification environment status (INT/PROD)
 - `pack_specification_version` - Pack specification version number
+- `pack_specification_billing_id` - Pack specification billing identifier
 - `supplier_pack_id` - Supplier pack ID
 - `supplier_pack_approval` - Supplier pack approval status
 - `supplier_pack_status` - Supplier pack environment status (INT/PROD)