[FEAT] Add enterprise billing usage data sources

[vmvarela]

Summary

This PR adds three new data sources for querying GitHub Enterprise billing
usage via the enhanced billing platform API:

• github_enterprise_billing_usage — itemized usage report
  (GET /enterprises/{enterprise}/settings/billing/usage)
• github_enterprise_billing_usage_summary — aggregated summary report
  (GET /enterprises/{enterprise}/settings/billing/usage/summary)
• github_enterprise_billing_premium_request_usage — premium request (e.g.
  Copilot) usage report
  (GET /enterprises/{enterprise}/settings/billing/premium_request/usage)

All three are read-only data sources. No resources are added.

Relates to #2739

Context

The enhanced billing platform endpoints for enterprises are not yet wrapped
by the go-github client library. This PR implements manual HTTP requests
using client.NewRequest / client.Do, following the same pattern used
elsewhere in the provider for endpoints not yet covered by the SDK.

Data Sources

github_enterprise_billing_usage

Returns itemized billing line items for an enterprise. Optional filters:
year, month, day, cost_center_id.

data "github_enterprise_billing_usage" "example" {
  enterprise_slug = "my-enterprise"
  year            = 2025
  month           = 6
}

github_enterprise_billing_usage_summary

Returns an aggregated summary of billing usage. Optional filters: year,
month, day, organization, repository, product, sku,
cost_center_id.

Note: This endpoint is in public preview and subject to change.

data "github_enterprise_billing_usage_summary" "example" {
  enterprise_slug = "my-enterprise"
  product         = "Actions"
}

github_enterprise_billing_premium_request_usage

Returns premium request usage (AI models, Copilot, etc.). Optional filters:
year, month, day, organization, user, model, product,
cost_center_id.

data "github_enterprise_billing_premium_request_usage" "example" {
  enterprise_slug = "my-enterprise"
  year            = 2025
  month           = 1
}

Testing

Acceptance tests are gated on skipUnlessMode(t, enterprise) and require
a GitHub Enterprise token (GITHUB_ENTERPRISE_SLUG, GITHUB_TOKEN).

Unit tests for all helper functions are in util_enterprise_billing_test.go
and run without any credentials.

Checklist

• Data source schema follows provider conventions
• ValidateDiagFunc used (not deprecated ValidateFunc)
• d.Set() errors checked and returned
• diag.Errorf used instead of diag.FromErr(fmt.Errorf(...))
• Data source IDs are stable (buildTwoPartID)
• Registered in provider.go and website/github.erb
• Documentation in website/docs/d/
• Unit tests pass (go test ./github/ -run TestFlatten...)

[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labeled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[vmvarela]

Updated acceptance tests (commit 9ebb722) to go beyond the trivial slug check:

• ID format assertion: StringRegexp verifies the ID is set with the correct slug:billing-usage / slug:billing-usage-summary / slug:billing-premium-request-usage format produced by buildTwoPartID
• usage_items is NotNull: verifies the list attribute is always populated (not nil/empty) after a successful read
• time_period is NotNull (summary + premium request data sources): verifies the block is returned by the API and set in state
• Additional filter sub-test for billing-usage (year+month combination)

All unit tests still pass (TestFlattenUsageItems, TestFlattenPremiumRequestUsageItems, TestFlattenUsageSummaryItems, TestFlattenTimePeriod).