chore(billing-usage): open upstream PR for enterprise_billing_usage data source

[vmvarela]

The billing-usage branch is complete and tested.

Tasks

• Review code against AGENTS.md Code Rules before opening PR
• Open PR against integrations/terraform-provider-github main branch
• Update this issue body with PR link and review status

Upstream PR

integrations#3293 — open, awaiting review.

Fixes applied before opening:

• Dereferenced *string pointer fields (organization_name, repository_name) in flattenUsageItems — were being stored as pointers in TypeString schema fields
• Replaced deprecated ValidateFunc with ValidateDiagFunc for year, month, day across all three data sources
• Replaced fork-local buildID() with upstream-compatible buildTwoPartID()

Branch

billing-usage

Resources implemented

• github_enterprise_billing_usage (data source)
• github_enterprise_billing_usage_summary (data source)
• github_enterprise_billing_premium_request_usage (data source)

[vmvarela]

Cerrando: el upstream PR integrations#3293 está abierto y el rebase con upstream/main completado. Todos los criterios de aceptación cumplidos.